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Molecular-orbital calculations on Pt2, PtH and PtCO with an optimized relativistic pseudopotential for Pt
Volume 129, number 6
CHEMICAL PHYSICS LETTERS
19 September 1986
MOLECULAR-ORBITAL CALCULATIONS ON Pt,, PtH AND PtCO WITH AN OPTIMIZED RELATIVISTIC PSEUDOPOTENTIAL FOR Pt A. GAVEZZOTTI, Dipartimento
G.F. TANTARDINI
di Chimica Fisica ed Elettrochimica
and M. SIMONETTA’ e Centro CNR, Universitb di Milano, Milan, Italy
Received 30 June 1986
A procedure for the derivation of an optimized relativistic pseudopotential for Pt is described and the results of MO calculations on simple molecules are given, The atomic properties of Pt are correctly reproduced by the pseudopotential. The Pt-H and Pt-C bonding are reasonably well described. A comparison is made with more sophisticated MO methods.
1. Introduction In the framework of our studies of the interaction of gas molecules with metal atom clusters [l-3], we report here the results of calculations performed to obtain an optimized relativistic peeudopotential (ORPP) for the Pt atom. The procedure consists of fitting the relativistic atomic wavefunctions of Pt by Gaussian functions (GTOs) and then using these orbitals in our standard treatment [3] to optimize the PP. Molecular calculations are then performed in the usual MO formalism, with a non-relativistic Hamiltonian. ln this paper the results for Pt2, PtH and PtCO are described and compared with available experimental and theoretical results to check the reliability of the ORPP. We discuss how this model passes the so-called “small molecule test” [4], a prerequisite for the application to large metal atom clusters; studies of such clusters with up to seven Pt atoms, using an optimized non-relativistic PP (OPP) and ORPP, as well as semiempirical schemes like extended Hiickel and relativistic extended Hiickel, will be reported elsewhere [5], with a critical examination of the relative performance of these methods.
Fock program of Desclaux [6] was used for a dgsl configuration. The resulting wavefunctions were adapted for use in MO GTO calculations as follows. The small components of the radial functions were omitted; the large components of p, d and f orbit& were spinaveraged with weights equal to the multiplicity of the j + l/2 andj - l/2 functions. The resulting radial function, in the form of a point mesh, was fitted to a nG expansion with n = 6 for Is, 2s, 3s, 4p, n = 7 for 4s, 5s, 5p, n = 5 for 2p, 3p, 3d, 4d, 4f. The number of Gaussians in each expansion was the minimum needed to reproduce the full nodal structure of each atomic function. The relativistic 6s and 5d valence functions were fitted to 3G and the pseudopotential expansion was optimized until correct atomic orbital energies were obtained. The 6p polarization function is so diffuse that it is insensitive to changes in the PP expansion and therefore a different optimization strategy was used. The PP expansion was kept equal to that of 6s, and the 2G expansion exponents were optimized instead, again until the correct value for the 6p atomic level was obtained in the selected state. The target values for the pseudopotential optimization were as follows: 6s
-0.2842 au
from relativistic calculation on dgsl. ,
5d
-0.4153 au
a weighted average of the two orbital energies (j+ l/2) from relativistic calculation on dgsl;
2. The relativistic Pt atom and ORPP To obtain relativistic Pt atomic orbitals, the Dirac’ Deceased 6 January 1986.
0 OO!?-2614/86$03.50 OElsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
577
CHEMICAL PHYSICS LETTERS
Volume 129, number 6
Table 1 Valence orbital basis set for Pt atom and Gaussian expansions of the corresponding pseudopotentiala)
6s
Orbital expansion
PP expansion
OLi
ci
ai
ci
2.3071 1.4890 0.0750
-1.24195 1.07076
149.81 31.15 10.70 5.25 2.21 1.33 1.02
0.15949 0.21816 0.18964 0.17617 0.14910 -0.03233 0.04558
148.33 31.12 10.79 5.01 2.55 2.15 0.94
0.16681 0.21986 0.20148 0.17374 0.03764 0.08962 0.01997
0.90093
5d
7.5183 1.1282 0.2830
-0.24578 0.64013 0.53590
6pb)
0.1919 0.0442
-0.20020 1.09312
-
a) The Gaussian expansions and the energies of the inner orbitals are available upon request. b) The PP expansion for 6p was set equal to that for 6s.
6~
-0.1277 au
tions, and a comparison with an accurate ab initio calculation by Wang and Pi&r (WP) [7]. It can be seen that the ORPP calculation gives (by construction) the exact atomic orbital energies, and also the correct order of stability for d9s1, dl” and d8s2 states of the Pt atom.
3. Molecular results Three electronic configurations were considered for Pt2 (fig. 1). The closed-shell configuration was found to dissociate to two dl” Pt atoms, as expected. The dissociation curves for the open-shell configurations were not calculated; table 3 shows the results at the bulk Pt interatomic distance, and a comparison with calculations by Basch, Cohen and Topiol @CT) [8] using a rigorously derived relativistic effective core potential [9] in an ab initio multiconfguration SCF framework. The electronic configuration and the binding energy curve for PtH are shown in fig. 2. The dissociation limit is, correctly, to dgsl Pt t 1s H. Table 4 shows our results for the single-configuration calculation together with results by WP [7] and BCT [8]. Three different electronic configurations (fig. 3) were tested for PtCO at the Pt-C distance of 1.852 A; for the most stable one, the binding energy curve was
a weighted average of the orbital energies for dgp1,2, dgp3,2 relativistic calculations.
The resulting valence orbital basis set and PP Gaussian expansion are reported in table 1. Table 2 gives the results for these atomic calcula-
19 September 1986
Pt - Pt
w
(SP)q,
Table 2 Results for the Pt atoma) ORPP d9s’ relative energy
e5d e6s ‘6~
(“0,
-0.4153 -0.2842 0.0526
WPb) d’s’
(-1 -0.4137 -0.2923 -
a) In parentheses data from ref. [7].
578
ORPP d”
0.0652 (0.0421) -0.2190 0.0617 0.0568
ORPP d’s2 sF
0.0568 (0.0225) -0.6189 -0.3291 0.0484
b, From ref. (71.
--!+-H--H-U -H-u-t+ +t*-!+
(1)
--!+-tl-H-+--t U-H-U -I+-I+-++
(2)
--H-U +-+-it +-!-l--Hu-l+-!+
(3)
Fig. 1. Molecular orbitals (schematic; the sequence of stability in the nb block is variable) and electron configurations for Pt2.
CHEMICAL PHYSICS LETTERS
Volume 129, number 6
19 September 1986
Table 4 Results for PtHa)
Table 3 Results for Pt2 Bond overlap population
Charge
s
P
ORPP ( 1)
0.416
0.057
0.201
ORPP (2)
0.938
0.097
0.694
ORPP (3)
0.888
0.075
0.664
BCT c, BCTd)
0.88 1.11
0.07 0.06
_
BEb)
-59.3 (22.5) -61.7 (20.1) 47.7 (129.5) 4.2 21.4
-
a) In ORPP calculations (l), (2), (3) refer to fig. 1; the Pt-Pt distance is fixed at the bulk value, 2.74 A. b) Binding energy in k&/mole; to d’s’, and in parentheses to dl” Pt; negative values are repulsive. C) Ref. [ 81 for a w-hole triplet state, at R, = 2.634 A, 3Xu. d) Ref. [8], S&hole singlet state, at R, = 2.574 A, ‘pg.
ORPPb) OPP c), WPd) exp. e, WPf) BCTg)
RI?
4.
WI?
1.43 1.83 1.61 1.53 1.59 1.472
71 14 56.5 79.3 55.8
2781 1200 2020 2294 2045 2288
a) R, in A, De in kcal/mole, we in cm-r. b) we computed after Morse interpolation. c, Ref. [5]. d, Ref. [7] for the 2A state. e, As quoted in ref. [ 71, for the 2A state. f) Ref. [7] for the 2I: state. g) Ref. [ 81 for the 2~ state. Pt-C-O
ud*
-
'd
-
upco’
-
f!+
a,co)
1 n
U~COIPt-H
”
-
-
+
-It +!--!t-H+-H-it -!4-it-tl-
+ ii--H--H-!+-!t-H-It-H--H-
-It * -it-l-+ -tt-tt-H-H-*+
0
+0.136
+0.424
(1)
(2)
(3)
Fig. 3. Molecular orbitals (schematic) and electronic confiiurations for PtCO. (1) corresponds to a IX+ closed-shell state, (2) to a 3Z+ triplet state; (3) is largely antibonding. Relative energies (in au) at R @‘t-C) = 1.852 A are given. E’(d”)
c
2.6
3.4
4.2
5.0
R(Pt-Ii), au. Fig. 2. Molecular orbit& (schematic), electron configurations and binding energy curve for PtH.
Fig. 4. Binding energy curve for closed-shell PEO, configuration (1) in fg. 3. R(Pt-C), a.u
Volume 129, number 6
19 September 1986
CHEMICAL PHYSICS LETTERS
Table 5 Results for Ptcoa)
R @t-C)
BCb) BC
‘IX+ 3z+
Overlap population
Charge
Pt-C
c-o
Pt(s)
C
0
Pt
1.905 1.905
0.347 0.697
1.142c) 1.144
0.539 -
0.072 -
-0.125 -
0.053 -
l.lOSd) 1.045 -
0.657 0.887 -
0.304
-0.328
0.024
-
-
1.101
0.122
0.34
-0.32
-0.02
ORPP ORPP ORPP
(1) (2) (3)
1.852 1.852 1.852
0.740 0.113 -0.391
OPPe)
(1)
1.852
0.137
Optimized CO distance Re
De(*‘O)
De(dg s1)
Bcb) BC ORPP OPPe)
lx+ 3z+ (1) (1)
1.707 1.824 1.91 2.40
70.3 72.0 9.4
42.7 19.4 27.0 -
HRf) HRg)
‘Z+ ‘Z+
2.073 1.977
19.8 44.5
-4.2 37.4
b, Ref. [lo]. c) 1.057 in free CO. *) 1.06 1 in free CO. a) R, R, in A, De in kcal/mole. g) Ref. [ 111, M$ller-Plesset II (MP2) results. f) Ref. [ 111, SCF results.
then calculated (fg 4). Table 5 shows the numerical detail, together with results from effective core potential ab initio MC SCF and CI calculations by Basch and Cohen (BC) [lo] or effective core potential SCF calculations by Rohlfmg and Hay (RI-I) [ 1I].
4. Analysis and conclusions The correct description of the Pt-Pt bonding is beyond the scope of our treatment. The most stable states for the Pt 2 molecule arise from &hole configurations [8], while our &hole configuration is strongly antibonding (see table 3). Of course the comparison between MS SCF and single-configuration calculations can only be an indirect one; also, the basis set for Pt in the BCT calculations is of double-zeta quality, while we use a minimal basis set. Our calculated dissociation energies are very large, and are presumably enhanced by an unknown amount of basis set superposition error (BSSE). Things look generally more favourable for PtH. The 2X and 2A states of this molecule are similar in stability [7,8], and we can compare results for the 580
e, Ref. [S].
2X state with our sigma-doublet single configuration (see table 4). The effects of the relativistically parameterized pseudopotential are evident on comparison of OPP and ORPP results. The ORPP equilibrium distance is much shortened, and compares favourably with those obtained by the much more rigorous treatments of WP and BCT. The value for the dissociation energy is quite close to the experimental one. For PtCO, comparisons can safely be conducted between our configuration (1) and the 1Z+ state of BC and RH. They both dissociate to dl” Pt (see fig. 4). The ORPP equilibrium Pt-C distance (see table 5) is 0.2 A longer than the MC SCF one, but competes with the MP2 one and is shorter than the SCF one in ref. [ 111. The dissociation energy (to dlo Pt) is also quite similar in ORPP and BC, while the difference in dissociation energy to d%l Pt reflects the different evaluation of the separation between the two atomic states. The Mulliken population analysis shows parallel trends on comparing BC and ORPP data; the s charge on Pt is 0.54 and 0.66 electron, respectively; the overall Pt charge is 0.053 and 0.024 electron; the C-O bond overlap population increases by 0.085 and 0.044.
Volume 129, number 6
CHEMICAL PHYSICS LETTERS
The carbon-oxygen charge separation is, on the other hand, rather different (about 0.3 in ORPP and about 0.1 in BC). We can offer at this point the following conclusions. When comparing sophisticated quantum chemical calculations with our parameterized ORPP, minimal basis set, single-configuration calculations, it is not easy to say which and how much of the discrepancies are due to which of the approximations. On the whole, we see that our method is inadequate to describe the metalmetal bond stretching, a stumbling block for even the most refined methods [8,12-141, but it fares reasonably well in the description of the energetic and electronic structure of small molecules like PtH and PtCO. Therefore, we feel encouraged to use our simple and computationally rather inexpensive procedure to calculate the electronic structure of large clusters interacting with small organic fragments [5], systems in which the quantum chemically more rigorous methods with multiple-zeta basis sets and electronic correlation become rapidly inapplicable.
19 September 1986
References 111 A. Gavezzotti, G.F. Tantardini and M. Simonetta, Chem. Phys. 84 (1984) 453.
[21 0. Bellezza, M.G. Cattania, A. Gavezzotti and M. Simonetta, Chem. Phys. Letters 108 (1984) 425.
131 A. Gavezzotti, G.F. Tantardini and M. Simonetta, Chem. Phys. 105 (1986) 333.
141 J. Andzelm and D.R. Salahub, Intern. J. Quantum Chem., to be published.
[Sl A. Gavezzotti, M. Miessner, M. Shnonetta and G.F. Tantardini, in preparation.
161 J.P. Desclauz, Computer Phys. Commun. 9 (1975) 31. [71 S.W. Wang and K.S. Pitzer, J. Chem. Phys. 79 (1983) 3851.
[81 H. Basch, D. Cohen and S. Topiol, Israel J. Chem. 19 (1980) 233.
(91 H. Basch and S. Topiol, J. Chem. Phys. 71 (1983) 809. [LOI H. Basch and D. Cohen, J. Am. Chem. Sot. 105 (1983) 3856.
[LLI C.M. Rohlfimg and P.J. Hay, J. Chem. Phys. 83 (1985) 4641.
[I21 1. Shim and K.A. Gingerich, J. Chem. Phys. 78 (1983) 5693.
[I31 I. Shim and K.A. Gingerich, J. Chem. Phys. 80 (1984) 5107.
1141 C.W. Bauschlicher Jr., S.P. Walch and P.E.M. Siegbahn, J. Chem. Phys. 76 (1982) 6015.
581
CHEMICAL PHYSICS LETTERS
19 September 1986
MOLECULAR-ORBITAL CALCULATIONS ON Pt,, PtH AND PtCO WITH AN OPTIMIZED RELATIVISTIC PSEUDOPOTENTIAL FOR Pt A. GAVEZZOTTI, Dipartimento
G.F. TANTARDINI
di Chimica Fisica ed Elettrochimica
and M. SIMONETTA’ e Centro CNR, Universitb di Milano, Milan, Italy
Received 30 June 1986
A procedure for the derivation of an optimized relativistic pseudopotential for Pt is described and the results of MO calculations on simple molecules are given, The atomic properties of Pt are correctly reproduced by the pseudopotential. The Pt-H and Pt-C bonding are reasonably well described. A comparison is made with more sophisticated MO methods.
1. Introduction In the framework of our studies of the interaction of gas molecules with metal atom clusters [l-3], we report here the results of calculations performed to obtain an optimized relativistic peeudopotential (ORPP) for the Pt atom. The procedure consists of fitting the relativistic atomic wavefunctions of Pt by Gaussian functions (GTOs) and then using these orbitals in our standard treatment [3] to optimize the PP. Molecular calculations are then performed in the usual MO formalism, with a non-relativistic Hamiltonian. ln this paper the results for Pt2, PtH and PtCO are described and compared with available experimental and theoretical results to check the reliability of the ORPP. We discuss how this model passes the so-called “small molecule test” [4], a prerequisite for the application to large metal atom clusters; studies of such clusters with up to seven Pt atoms, using an optimized non-relativistic PP (OPP) and ORPP, as well as semiempirical schemes like extended Hiickel and relativistic extended Hiickel, will be reported elsewhere [5], with a critical examination of the relative performance of these methods.
Fock program of Desclaux [6] was used for a dgsl configuration. The resulting wavefunctions were adapted for use in MO GTO calculations as follows. The small components of the radial functions were omitted; the large components of p, d and f orbit& were spinaveraged with weights equal to the multiplicity of the j + l/2 andj - l/2 functions. The resulting radial function, in the form of a point mesh, was fitted to a nG expansion with n = 6 for Is, 2s, 3s, 4p, n = 7 for 4s, 5s, 5p, n = 5 for 2p, 3p, 3d, 4d, 4f. The number of Gaussians in each expansion was the minimum needed to reproduce the full nodal structure of each atomic function. The relativistic 6s and 5d valence functions were fitted to 3G and the pseudopotential expansion was optimized until correct atomic orbital energies were obtained. The 6p polarization function is so diffuse that it is insensitive to changes in the PP expansion and therefore a different optimization strategy was used. The PP expansion was kept equal to that of 6s, and the 2G expansion exponents were optimized instead, again until the correct value for the 6p atomic level was obtained in the selected state. The target values for the pseudopotential optimization were as follows: 6s
-0.2842 au
from relativistic calculation on dgsl. ,
5d
-0.4153 au
a weighted average of the two orbital energies (j+ l/2) from relativistic calculation on dgsl;
2. The relativistic Pt atom and ORPP To obtain relativistic Pt atomic orbitals, the Dirac’ Deceased 6 January 1986.
0 OO!?-2614/86$03.50 OElsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
577
CHEMICAL PHYSICS LETTERS
Volume 129, number 6
Table 1 Valence orbital basis set for Pt atom and Gaussian expansions of the corresponding pseudopotentiala)
6s
Orbital expansion
PP expansion
OLi
ci
ai
ci
2.3071 1.4890 0.0750
-1.24195 1.07076
149.81 31.15 10.70 5.25 2.21 1.33 1.02
0.15949 0.21816 0.18964 0.17617 0.14910 -0.03233 0.04558
148.33 31.12 10.79 5.01 2.55 2.15 0.94
0.16681 0.21986 0.20148 0.17374 0.03764 0.08962 0.01997
0.90093
5d
7.5183 1.1282 0.2830
-0.24578 0.64013 0.53590
6pb)
0.1919 0.0442
-0.20020 1.09312
-
a) The Gaussian expansions and the energies of the inner orbitals are available upon request. b) The PP expansion for 6p was set equal to that for 6s.
6~
-0.1277 au
tions, and a comparison with an accurate ab initio calculation by Wang and Pi&r (WP) [7]. It can be seen that the ORPP calculation gives (by construction) the exact atomic orbital energies, and also the correct order of stability for d9s1, dl” and d8s2 states of the Pt atom.
3. Molecular results Three electronic configurations were considered for Pt2 (fig. 1). The closed-shell configuration was found to dissociate to two dl” Pt atoms, as expected. The dissociation curves for the open-shell configurations were not calculated; table 3 shows the results at the bulk Pt interatomic distance, and a comparison with calculations by Basch, Cohen and Topiol @CT) [8] using a rigorously derived relativistic effective core potential [9] in an ab initio multiconfguration SCF framework. The electronic configuration and the binding energy curve for PtH are shown in fig. 2. The dissociation limit is, correctly, to dgsl Pt t 1s H. Table 4 shows our results for the single-configuration calculation together with results by WP [7] and BCT [8]. Three different electronic configurations (fig. 3) were tested for PtCO at the Pt-C distance of 1.852 A; for the most stable one, the binding energy curve was
a weighted average of the orbital energies for dgp1,2, dgp3,2 relativistic calculations.
The resulting valence orbital basis set and PP Gaussian expansion are reported in table 1. Table 2 gives the results for these atomic calcula-
19 September 1986
Pt - Pt
w
(SP)q,
Table 2 Results for the Pt atoma) ORPP d9s’ relative energy
e5d e6s ‘6~
(“0,
-0.4153 -0.2842 0.0526
WPb) d’s’
(-1 -0.4137 -0.2923 -
a) In parentheses data from ref. [7].
578
ORPP d”
0.0652 (0.0421) -0.2190 0.0617 0.0568
ORPP d’s2 sF
0.0568 (0.0225) -0.6189 -0.3291 0.0484
b, From ref. (71.
--!+-H--H-U -H-u-t+ +t*-!+
(1)
--!+-tl-H-+--t U-H-U -I+-I+-++
(2)
--H-U +-+-it +-!-l--Hu-l+-!+
(3)
Fig. 1. Molecular orbitals (schematic; the sequence of stability in the nb block is variable) and electron configurations for Pt2.
CHEMICAL PHYSICS LETTERS
Volume 129, number 6
19 September 1986
Table 4 Results for PtHa)
Table 3 Results for Pt2 Bond overlap population
Charge
s
P
ORPP ( 1)
0.416
0.057
0.201
ORPP (2)
0.938
0.097
0.694
ORPP (3)
0.888
0.075
0.664
BCT c, BCTd)
0.88 1.11
0.07 0.06
_
BEb)
-59.3 (22.5) -61.7 (20.1) 47.7 (129.5) 4.2 21.4
-
a) In ORPP calculations (l), (2), (3) refer to fig. 1; the Pt-Pt distance is fixed at the bulk value, 2.74 A. b) Binding energy in k&/mole; to d’s’, and in parentheses to dl” Pt; negative values are repulsive. C) Ref. [ 81 for a w-hole triplet state, at R, = 2.634 A, 3Xu. d) Ref. [8], S&hole singlet state, at R, = 2.574 A, ‘pg.
ORPPb) OPP c), WPd) exp. e, WPf) BCTg)
RI?
4.
WI?
1.43 1.83 1.61 1.53 1.59 1.472
71 14 56.5 79.3 55.8
2781 1200 2020 2294 2045 2288
a) R, in A, De in kcal/mole, we in cm-r. b) we computed after Morse interpolation. c, Ref. [5]. d, Ref. [7] for the 2A state. e, As quoted in ref. [ 71, for the 2A state. f) Ref. [7] for the 2I: state. g) Ref. [ 81 for the 2~ state. Pt-C-O
ud*
-
'd
-
upco’
-
f!+
a,co)
1 n
U~COIPt-H
”
-
-
+
-It +!--!t-H+-H-it -!4-it-tl-
+ ii--H--H-!+-!t-H-It-H--H-
-It * -it-l-+ -tt-tt-H-H-*+
0
+0.136
+0.424
(1)
(2)
(3)
Fig. 3. Molecular orbitals (schematic) and electronic confiiurations for PtCO. (1) corresponds to a IX+ closed-shell state, (2) to a 3Z+ triplet state; (3) is largely antibonding. Relative energies (in au) at R @‘t-C) = 1.852 A are given. E’(d”)
c
2.6
3.4
4.2
5.0
R(Pt-Ii), au. Fig. 2. Molecular orbit& (schematic), electron configurations and binding energy curve for PtH.
Fig. 4. Binding energy curve for closed-shell PEO, configuration (1) in fg. 3. R(Pt-C), a.u
Volume 129, number 6
19 September 1986
CHEMICAL PHYSICS LETTERS
Table 5 Results for Ptcoa)
R @t-C)
BCb) BC
‘IX+ 3z+
Overlap population
Charge
Pt-C
c-o
Pt(s)
C
0
Pt
1.905 1.905
0.347 0.697
1.142c) 1.144
0.539 -
0.072 -
-0.125 -
0.053 -
l.lOSd) 1.045 -
0.657 0.887 -
0.304
-0.328
0.024
-
-
1.101
0.122
0.34
-0.32
-0.02
ORPP ORPP ORPP
(1) (2) (3)
1.852 1.852 1.852
0.740 0.113 -0.391
OPPe)
(1)
1.852
0.137
Optimized CO distance Re
De(*‘O)
De(dg s1)
Bcb) BC ORPP OPPe)
lx+ 3z+ (1) (1)
1.707 1.824 1.91 2.40
70.3 72.0 9.4
42.7 19.4 27.0 -
HRf) HRg)
‘Z+ ‘Z+
2.073 1.977
19.8 44.5
-4.2 37.4
b, Ref. [lo]. c) 1.057 in free CO. *) 1.06 1 in free CO. a) R, R, in A, De in kcal/mole. g) Ref. [ 111, M$ller-Plesset II (MP2) results. f) Ref. [ 111, SCF results.
then calculated (fg 4). Table 5 shows the numerical detail, together with results from effective core potential ab initio MC SCF and CI calculations by Basch and Cohen (BC) [lo] or effective core potential SCF calculations by Rohlfmg and Hay (RI-I) [ 1I].
4. Analysis and conclusions The correct description of the Pt-Pt bonding is beyond the scope of our treatment. The most stable states for the Pt 2 molecule arise from &hole configurations [8], while our &hole configuration is strongly antibonding (see table 3). Of course the comparison between MS SCF and single-configuration calculations can only be an indirect one; also, the basis set for Pt in the BCT calculations is of double-zeta quality, while we use a minimal basis set. Our calculated dissociation energies are very large, and are presumably enhanced by an unknown amount of basis set superposition error (BSSE). Things look generally more favourable for PtH. The 2X and 2A states of this molecule are similar in stability [7,8], and we can compare results for the 580
e, Ref. [S].
2X state with our sigma-doublet single configuration (see table 4). The effects of the relativistically parameterized pseudopotential are evident on comparison of OPP and ORPP results. The ORPP equilibrium distance is much shortened, and compares favourably with those obtained by the much more rigorous treatments of WP and BCT. The value for the dissociation energy is quite close to the experimental one. For PtCO, comparisons can safely be conducted between our configuration (1) and the 1Z+ state of BC and RH. They both dissociate to dl” Pt (see fig. 4). The ORPP equilibrium Pt-C distance (see table 5) is 0.2 A longer than the MC SCF one, but competes with the MP2 one and is shorter than the SCF one in ref. [ 111. The dissociation energy (to dlo Pt) is also quite similar in ORPP and BC, while the difference in dissociation energy to d%l Pt reflects the different evaluation of the separation between the two atomic states. The Mulliken population analysis shows parallel trends on comparing BC and ORPP data; the s charge on Pt is 0.54 and 0.66 electron, respectively; the overall Pt charge is 0.053 and 0.024 electron; the C-O bond overlap population increases by 0.085 and 0.044.
Volume 129, number 6
CHEMICAL PHYSICS LETTERS
The carbon-oxygen charge separation is, on the other hand, rather different (about 0.3 in ORPP and about 0.1 in BC). We can offer at this point the following conclusions. When comparing sophisticated quantum chemical calculations with our parameterized ORPP, minimal basis set, single-configuration calculations, it is not easy to say which and how much of the discrepancies are due to which of the approximations. On the whole, we see that our method is inadequate to describe the metalmetal bond stretching, a stumbling block for even the most refined methods [8,12-141, but it fares reasonably well in the description of the energetic and electronic structure of small molecules like PtH and PtCO. Therefore, we feel encouraged to use our simple and computationally rather inexpensive procedure to calculate the electronic structure of large clusters interacting with small organic fragments [5], systems in which the quantum chemically more rigorous methods with multiple-zeta basis sets and electronic correlation become rapidly inapplicable.
19 September 1986
References 111 A. Gavezzotti, G.F. Tantardini and M. Simonetta, Chem. Phys. 84 (1984) 453.
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