An ab initio study of the intermolecular potential surfaces of HeCH4 and NeCH4

17 October 1997

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 277 (1997) 483-489

An ab initio study of the intermolecular potential surfaces of H e - C H 4 and N e - C H 4 Daqing Gao a, Liangjin Chen a, Zhiru Li a,1, Fu-Ming Tao b, Yuh-Kang Pan a a Department of Chemistry, Boston College, Chestnut Hill, MA 02167, USA b Department of Chemistry and Biochemistry, California State Universi~.,, Fullerton, CA 92834, USA Received 28 May 1997; in final form 28 July 1997

Abstract The intermolecular potential surfaces for the Van der Waals complexes He-CH 4 and Ne-CH 4 are studied by ab initio theory using complete fourth-order M¢ller-Plesset perturbation theory (MP4) with an efficient basis set containing bond functions. For He-CH 4, the global minimum occurs at R = 3.4 .&, 0 = 180°, ~b= 0 ° (in a Jacobi coordinate system) with a minimum energy De = - 2 6 . 2 c m - f . For Ne-CH 4, the global minimum occurs at R = 3.5 A, 0 = 180°, ~b= 0° with a minimum energy De = - 59.0 c m - t. Two saddle points were found for each of the systems. The potential energy surfaces are compared with the semiempirical surfaces and recent ab initio results on the same systems. @ 1997 Elsevier Science B.V.

1. Introduction Accurate determination of the intermolecular forces of Van der Waals complexes represents a great challenge in ab initio molecular orbital calculations [1,2]. The dominant contribution of the Van der Waals interaction energies is dispersion attraction; however, recovery of the dispersion attraction requires a large number of polarization functions such as 5d4f3g2hli and a high level of electron correlation or configuration interaction treatments [3]. Given the present computational resources, the costs of such computation are formidable. Recently, we proposed an efficient approach which uses adequately

On leave from Department of Chemistry, Jilin University, Changchun, Jilin, China.

large atomic basis sets in the core and valence orbital space (s,p part) combined with a set of bond functions [4]. The bond functions were shown to be more effective than polarization functions in recovering the intermolecular correlation energy or dispersion energy. The significance of using bond functions is that it allows very accurate calculations with the size of the overall basis set controlled to a manageable level, making high-order polarization functions beyond the f-type are no longer necessary. A small number o f low polarization functions are, however, still needed for the adequate description of intra-systern electron correlation and monomer properties [5]. The efficiency and reliability of the method have been demonstrated in a series of applications ranging from rare gas dimers to ammonia dimers [6-10]. The approach has already gained popularity among other

0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0009-26 14(97)00938-X

D. Gao et aL/ ChemicalPhysics Letters 277 (1997) 483-489

484

research groups [11-13]. In this paper, we extend our explorations to the systems of methane with helium and neon. Methane is regarded as one of the origin gases of life on the Earth or in the universe. There have been great interest in the complexes of methane with rare gas atoms. Experimental studies of these complexes have been done by thermal diffusion [14], by rotationally inelastic scattering [15], and by crossed molecular beam experiments [16,17]. Since most of the physical properties of the rare gas-methane systems are dependent on the intermolecular potentials, an accurate determination of the complete anisotropic potential energy surfaces of H e - C H 4, N e - C H 4 are urgently needed. There appears to be no high level ab initio molecular orbital calculations for the H e CH4, N e - C H 4 systems. Szczesniak et al. studied the A r - C H 4 complex with ab initio calculations and recovered 75% of the well depth [18]. Yin and MacKerell performed calculations on the H e - C H 4 and N e - C H 4 systems at the MP3/6-311 + + G(3d,3p) level [19]. Semiempirical potential energy surfaces were determined for the H e - C H 4 [16], N e C H 4 [17], and A r - C H 4 [20] systems over a decade ago by Buck et al. These potential energy surfaces were based on the models of self-consistent Hartree-Fock plus damped dispersion energy. In their potentials, the same face configuration in which the noble gas is along the C3v axis facing the three hydrogen atoms was found to be the global minimum for the systems of H e - C H 4 and A r - C H 4. For the N e - C H 4 system, the edge configuration in which the Ne atom is located on the bisector of the angle between the two H - C bonds in the H - C - H plane was found to be the global minimum [21]. The equilibrium distances are, RHe_Cr~4 = 3.4 A, RNe_CH4 = 3.6 A, and RAr_CH4 = 3.5 A. The well depths are, VHe_CH,= --27.0 cm -1, VNe_C~, = --53.2 cm -1, and VAr_CrL= --152.5 cm -1. The anisotropy part of the potential energy surfaces was approximated by a damping function which was indicated to be insufficient; Further refinement of these potentials is warranted [15]. Thus, in this study, we update the complete potential energy surfaces of H e - C H 4 and N e - C H 4 using complete fourth-order M¢ller-Plesset perturbation (MP4) theory [22] with an efficient basis set incorporating bond functions. The rare gas atom may be regarded as a structureless

probe of the molecular force field around the methane molecule. Therefore, this work will provide some fresh insights into the structure and interaction energies of the systems.

2. Computational details 2.1. Coordinate systems The intermolecular potential surfaces for H e - C H 4 and N e - C H 4 are expressed in a body-fixed Jacobi coordinate system with the origin at the center of the mass of C H 4 which is the carbon atom. The internal coordinates of C H 4 a r e held fixed at the experimental equilibrium bond length rcn = 1.092 A and bond angle / _ H C H = 109.466 ° [23]. The intermolecular coordinates of H e - C H 4 and N e - C H 4 are described as follows. R is the distance between He or Ne and the carbon atom of C H 4 ; 0 (polar angle) is the angle between the R vector and C3v axis of C H 4 along one C - H bond pointing away from the carbon atom, and ~ is the angle for out-of-plane rotation about the symmetry axis from a reference geometry (th = 0 °) with He or Ne, C, and two H atoms coplanar with

,

:

¢

No"

z .......

o

He

face configuration ( 0 = 180 °, ~b= 0 ° )

r'~"~/tl

o

,,,,,,, .......

edge configuration

He

( 0 = 54.733 °, ¢ = 0 ° )

x•,,•. - -

He

vertex configuration ( 0 = 0 °, * = 0 ° )

Fig. 1. T h e i n t e r m o l e c u l a r c o o r d i n a t e s f o r the H e - C H 4 s y s t e m a n d three u n i q u e c o n f i g u r a t i o n s .

D. Gao et al. / Chemical Physics Letters 277 (1997) 483-489

the axis. Fig. 1 shows the intermolecular coordinates for H e - C H 4 along with three unique configurations. The three configurations are the face configuration (0 = 180 °, ~b = 0°), edge configuration (0 = 54.733 °, ~b = 0°), and vertex configuration (0 = 0 °, ¢b = 0°).

485

error (BSSE) [26]. The validity of the CP method is supported by a series of recent successful applications. All the calculations were carried out using the Gaussian 94 package [27]. 2.3. Basis set

2.2. Theoretical method

It is well known that the Hartree-Fock interaction is repulsive and the dispersion energy is the dominant attractive intermolecular force in a rare-gas complex. It is thus essential to calculate the X - C H 4 (X = He, Ne) intermolecular potential at a highly correlated level. In the present calculations we use the complete fourth-order M¢ller-Plesset perturbation method (MP4). Several studies have shown that the MP4 method is capable of recovering over 95% of the correlation contribution to the intermolecular energy, which is comparable to the single and double excitation coupled cluster theory including all connected triple excitations in a noniterative manner (CCSD(T)) [24,251. The intermolecular energy, A E, is the difference between the total energy of the complex, EX_CH4, and the sum of the monomer energies, UGH4 -~-Ex (X = He or Ne). The full counterpoise (CP) method was applied to correct for the basis set superposition

As in our calculation of other systems, the basis set for He or N e - C H 4 in the present work consists of the nucleus-centered part and the bond function set 3s3p2d. The nucleus-centered part is a very large set of sp Gaussian basis functions on C and Ne atoms (s functions on He and H) extended with a small number of polarization functions. Specifically, the sp set for the C atom is [6s4p], contracted from the well-tempered [14s9p] of Huzinaga et al. [28], the sp set for Ne is [8s5p], contracted from the well-tempered [14sl0p] [29,30], and the s set for He and H is [5s], contracted from the well-tempered [10s] of Huzinaga [31]. The basis set is extended with different polarization functions for each atom with the Gaussian exponents listed in Table 1. The bond functions 3s3p2d are Gaussian primitives (exponent values: c~S = Ctp = 0.9, 0.3, 0.1 and a d = 0.6, 0.2) centered at the midpoint between He or Ne and the C atom of CH 4. The same bond functions are universally appropriate for different intermolecular systems, mainly because intermolecular energies are quickly saturated by the bond functions.

Table 1 Basis set and basis functions Location

Basis functions

3. Results and discussion

at nucleus C

H He Ne

14s9p contracted to 6s4p [28] 3d functions with exponents otd = 1.252, 0.313, 0.07825 10s contracted to 5s [31] 3p with otp = 1.5, 0.375, 0.09375 10s contracted to 5s [31] 3p with ap = 2.56, 0.64, 0.16 14sl0p contracted to 8s5p [29,30] 2d with a d = 1.35, 0.45 I f with tel = 0.56

at midbond R c = 0.5R

3s ( a s = 0.9, 0.3, 0.1) 3p (Otp = 0.9, 0.3, 0.1) 2d ( a d = 0.6, 0.2)

3.1. H e - C H 4 surface

Tables 2 and 3 list the intermolecular energies of H e - C H 4 calculated at the SCF, MP2, and MP4 levels for various values of the intermolecular coordinates (R, 0, ~b). The polar angle 0 varies from 0 °, 30 °, 54.733 °, 60 °, 120 °, 150°, and 180 ° for each of the three values of the out-of-plane rotation angle, ~b = 0 °, 30 °, 60 °. For a given orientation, the intermolecular separation R varies from 3.0 to 5.0 ~, in variable intervals covering the potential surface from the repulsive wall to the attractive tail region. Figs. 2 and 3 show the angular dependence of the potential at the fixed distance, R = 3.4 A, and along the minimum path R = R m , respectively. Due to the

486

D. Gao et a L / Chemical Physics Letters 277 (1997) 483-489

high symmetry presented

20L~

of the methane molecule, results are

o n l y f o r ~b = 0 ° a n d

0 f r o m 0 ° to 3 6 0 °.

V

/~V

The discussions will be focused on the MP4 results.

/

,

/

!

T h e S C F a n d M P 2 r e s u l t s will b e b r i e f l y d i s c u s s e d at

'

/ /i

the end of this section. It is c l e a r f r o m potential

minimum

ir

Tables

2 and

occurs

near

180 °, 4 ) = 0°), c o r r e s p o n d i n g tion with a well depth of experimental cm -j We

3 t h a t ao g l o b a l (R = 3.4 A,

/

0=

to t h e f a c e c o n f i g u r a -

D e = -26.2

well °depth of Buck

/

E

-27.0

/

f

,

cm -1. The

et al. is

,

-10 r

"",

,\/ E

-2oi

:4

\

~

160

210

//

z

<

"-.,~;

at R = 3 . 4 A w i t h f a c e c o n f i g u r a t i o n [ 1 6 , 2 1 ] . would

expect

underestimated performance

based

energy

D e to b e

on the consistent

10

60

110

[4], H e - C O

[32], H e - H C N

[33],

[10]. A s a r e s u l t , t h e t r u e w e l l d e p t h f o r t h e

260

310

360

0 / deg

of the method on other similar systems

s u c h as H e - H e He-H20

the minimum

by 5-8%,

Fig. 2. Angular dependence of the intermolecular energy A E(MP4) for the H e - C H 4 at R = 3.6 ,~ and q~ = 0 °.

He-CH

4 interaction potential

is l i k e l y to b e n e a r

Table 2 Intermolecular energies (in c m - J ) of H e - C H 4 calculated at SCF, MP2, and MP4 levels for q5 = 0 °

D e = -28

R (~,)

b a t i o n t h e o r y at t h e f o u r t h - o r d e r f o r e l e c t r o n c o r r e l a -

0 0o

3.0

3.2

30 °

54.733 ° 60 °

120 °

150°

180°

HF 709.76 399.84 220.65 229.32655.81 258.14 114.91 MP2533.89 264.33 110.93 118.25477.52 143.49 23.65 MP4505.63 240.89 91.44 98.60459.45 124.13 9.37 HF 341.07 192.44 105.46 109.69315.41 123.74 53.90 MP2224.62 100.95 30.05 33.45203.35 45.46 - 9 . 4 2 MP4203.87 84.31 16.57 19.78183.14 31.85-19.41

3.4

HF 162.58 MP2 84.61 MP4 69.65

91.89 50.09 52.11 150.45 58.87 25.17 29.44 - 2 . 2 8 - 0 . 7 5 75.07 4.87-19.26 1 7 . 7 6 - 1 1 . 5 9 - 1 0 . 2 1 60.72 - 4 . 6 1 - 2 6 . 1 6

3.6

HF 76.92 43.60 23.66 24.61 71.28 27.86 11.62 MP2 23.97 0.37 - 13.08 - 12.45 19.91 - 9 . 8 6 - 19.73 MP4 13.45 - 7 . 0 8 - 19.51 - 18.97 9 . 8 3 - 16.44 - 2 4 . 5 9

3.8

HF 38.46 20.57 11.13 11.59 33.56 13.11 5.39 MP2 - 0 . 2 6 - 9 . 0 7 - 14.95 - 14.69 - 2 . 7 1 - 13.54 - 17.11 MP4 - 7 . 5 9 - 15.30 - 19.43 - 19.25 - 8 . 9 1 - 18.15 -20.51

4.0

HF 16.95 9.66 5.22 5.42 15.74 6.12 2.46 MP2 8.53 - 1 1.79 - 13.52 - 13.41 - 9.09 - l 2.93 - 13.81 MP4 - 13.55- 15.72 - 16.66 - 16.62 - 13.96- 16.16 - 18.00

c m - 1 . T h e m a i n s o u r c e o f e r r o r in o u r

c a l c u l a t i o n s m a y b e d u e to t r u n c a t i o n o f t h e p e r t u r tion. For He-He,

recent studies [11,24,25] show that

the underestimate

of MP4

w e l l d e p t h is c o m p l e t e l y

recovered by full CI theory. Two

saddle

points

are

surface. One corresponds

found

(0=

5 4 . 7 3 3 °, & = 0 °) w i t h a w e l l d e p t h o f 1 at

R=3.6

global minimum.

A

and

is 6 . 7

-2o [ >

HF 0.37 0.20 0. I1 0.11 0.33 0.13 0.05 MP2-4.92 -4.50 -4.21 -4.22 -4.87 -4.22 -3.87 MP4-5.86 -5.27 -4.87 -4.90-5.77 -4.87 -4.40

-19.5

r above

Another corresponds

the

to t h e v e r t e x

I ' '-'

'

'~

E

F

-25i -30

P ~ 10

5.0

cm

I

-

HF 2.46 1.45 0.77 0.81 2.33 0.90 0.37 MP2-8.63 -8.21 -7.88 -7.88-8.52 -7.86 -7.33 MP4 - 10.73 - 9.83 - 9.26 - 9.31 - 10.56 - 9.24 - 8.43

the potential

cm

' ' ' ' I ....

4.5

on

to t h e e d g e c o n f i g u r a t i o n

'

I , , ~ , I ,

60

110

160

210

, , r

260

310

!

360

0 / deg Fig. 3. Angular dependence of the minimum intermolecular energy for H e - C H 4.

D. Gao et al. / Chemical Physics Letters 277 (1997) 483-489

configuration (0 = 0 °, ~b = 0 °) with a well depth of - 1 3 . 6 cm 1 at R = 4 . 0 A andis 12.6 cm -1 above the global minimum. The experimental well depths of the edge and vertex configurations a r e Vedg e = - 2 5 . 1 cm -~ at R = 3.6 A, Vvertex ~-- - - 1 4 . 4 cm -1 at R = 4.1 A. Thus, the semiempirical potentials of the H e - C H 4 system of Buck et al. are in good agreement with our present results using high level ab initio calculations. Like most other He complexes, the H e - C H 4 complex is very weakly bounded, its potential energy surface is highly anisotropic, and there is substantial radial-angular coupling in the potential. These are shown from Figs. 2 and 3. For instance, at short distances, such as R = 3.2 A, the potential is the most repulsive at the vertex configu-

487

ration, and is the least repulsive at the face configuration. At long distances, such as R = 4.0 A, the potential is most attractive at the face configuration. The vertex configuration coincides with a linear hydrogen-bonded orientation, while the face configuration corresponds to a symmetrical orientation (C 3v) midway between three hydrogen atoms. This is in consistent with our recent result on the H e - H 2 0 system that the global minimum geometry was found to be drastically bent from linear hydrogen-bonded configuration [10]. Recently, Yin and Mackerell performed an ab initio calculation on the three main configurations of H e - C H 4 and N e - C H 4, as well as He-He, Ne-Ne, and C H 4 - C H 4 with the goal of obtaining accurate

Table 3 I n t e r m o l e c u l a r e n e r g i e s (in c m - 1 ) o f H e - C H 4 c a l c u l a t e d at S C F , M P 2 , a n d M P 4 l e v e l s f o r d~ = 3 0 °, 6 0 ° R (~,)

~h = 30 ° 0=30 °

3.0

3.2

3.4

3.6

4.5

5.0

°

0=

150 °

0=30 °

0=60 °

0=

120 °

0=150

395.27

175.95

411.71

217.00

390.79

129.40

218.41

178.07

MP2

260.49

73.82

274.61

108.53

256.76

35.64

109.05

75.61

MP4

237.42

56.55

250.96

90.21

234.04

20.67

89.69

58.23

HF

190.23

83.66

198.16

103.71

188.01

60.96

104.34

84.72

MP2

99.26

13.24

105.70

29.25

97.59

- 4.03

29.25

14.08

MP4

82.84

1.25

88.90

16.48

81.42

- 14.44

15.75

1.94

HF

90.81

39.53

94.61

49.19

89.79

28.57

49.55

40.03

MP2

28.71

- 9.51

31.55

- 2.36

28.05

- 16.85

- 2.67

- 9. l 6

MP4

17.21

- 15.91

19.73

- 11.25

16.64

-24.07

- 11.88

- 17.53

HF

43.07

18.59

44.84

MP4

4.0

0=120

HF

MP2

3.8

~b = 6 0 ° 0=60 °

HF

0.13 -7.88

-21.67

-6.91

42.54

13.29

23.39

18.84

-0.16

-18.79

-13.19

-15.75

- 19.10

-8.06

-23.86

- 19.68

-21.61

6.18

10.99

8.84

- 15.81

-9.31

- 14.76

-9.84

- 16.80

- 14.97

- 15.80

MP4

- 15.32

- 19.89

- 14.97

- 19.05

- 15.33

-20.36

- 19.41

- 19.83

2.83

5.13

4.13

- 13.65

- 11.67

- 13.29

- 11.77

- 13.76

- 13.50

- 13.66

MP4

- 15.67

- 16.51

- 15.65

- 16.38

- 15.65

- 16.27

- 16.69

- 16.54

0.62

1.47

5.13

20.25

- 11.79

4.90

9.94

10.94

MP2

HF

4.06

21.19

73.18 -12.87

-9.76

9.15

8.69

1.31

MP2

HF

20.33

-15.91

0.73

9.42

1.41

0.42

0.73

0.59

MP2

-8.17

-7.68

-8.21

-7.74

-8.16

-7.43

-7.85

-7.66

MP4

-16.78

-8.89

-9.90

-9.05

-9.79

-8.48

-9.26

-8.93

HF MP2

0.20 -4.48

0.09 -4.08

0.22 -4.55

0.11 -4.13

0.37 -4.45

0.21 -3.95

0.11 -4.24

0.09 -4.08

MP4

-5.22

-4.68

-5.29

-4.76

-5.18

-4.50

-4.81

-4.68

°

488

D. Gao et al. / Chemical Physics Letters 277 (1997) 483-489

Lennard-Jones parameters in the parametrizations of empirical force fields for molecular simulation studies in the condensed phase [19]. For H e - C H 4, at the MP3/6-31G(3d,3p) level with BSSE correction, they obtained the well depth Vface = - 19.1 c m - 1 for the face configuration at R = 3.5 A, V~dge = - 14.3 cm -j for the edge configuration at R = 3.8 A, Vvertex = - - 11.4 c m - J for the vertex configuration at R = 4.2 A. At the M P 3 / 6 - 3 1 1 + + G(3d,3p) level without BSSE correction, they obtained Vface= 1 o -29.0cmat R = 3 . 4 A, V~dg~ = - 2 1 . 5 cm -1 at R = 3.7 A, and V~tex = - 22.4 c m - l at R = 4.2 A. It can be seen from these data and the analysis in their paper, that the minimum interaction energies are underestimated and the minimum distances are overestimated at the MP3/6-31G(3d,3p) level with BSSE corrections; the interaction energies are significantly more favorable at the MP3/6-311 + + G(3d,3p) level, and the distances may be considered close to the experiment. At the MP3/6-311 + + G(3d,3p) level, the minimum interaction energy for the vertex configuration is predicted to be more favorable than the edge configuration which is in contrast to our computational results that the edge minimum is more favorable than the vertex minimum. Any dramatic differences between the two levels are seen for the N e - C H 4 system in the next section. The SCF interaction energies are all repulsive. This confirms that the H e - C H 4 complex is bound dominantly by dispersion forces. The MP2 potential energy surface exhibits the same major features as the MP4 potential energy surface. However, the MP2 equilibrium distance, R m = 3 . 8 ,~, is 0.2 ,~ longer than the MP4 value, and the well depth, D e = - 2 0 . 5 cm -~, is only 78% of the MP4 value. In general, the MP2 potential is shallower than the MP4 potential, and the anisotropy of the MP2 potential is proportionally smaller.

3.2. Ne-CH 4 surface We also calculated the three main configurations of the N e - C H 4 system. Table 4 lists the computational results. We obtained the global potential minimum near R = 3.5 A for the face configuration with the well depth Vface= - 5 9 . 0 cm-1 at the MP4 level and Vedg e ~ - 4 3 . 7 cm-1 near R = 3.5 A, Vvertex =

Table 4 I n t e r m o l e c u l a r e n e r g i e s (in c m - i ) o f N e - C H 4 c a l c u l a t e d at S C F , M P 2 , and M P 4 l e v e l s for the three u n i q u e c o n f i g u r a t i o n s o f the system R (,~) 3.0

3.5

4.0

4.5

5.0

5.5

6.0

Face HF

Edge

Vertex

254.80

466.23

1433.23

MP2

61.68

230.09

1070.95

MP4

26.58

184.89

1011.25 220.23

37.31

69.80

MP2

HF

- 44.45

- 24.89

82.53

MP4

- 59.02

- 43.70

53.84

5.20

10.27

32.62

MP2

HF

- 30.49

- 30.55

- 22.89

MP4

- 36.54

- 38.30

- 34.83

0.72

1.49

4.74

MP2

HF

- 16.86

- 17.34

- 19.40

MP4

- 18.72

- 20.68

- 24.36

HF

0.11

0.22

0.68

MP2

-8.41

-9.13

- 10.76

MP4

-9.66

- 10.67

- 12.91

0.02

0.02

0.11

MP2

HF

- 4.59

- 4.92

- 5.72

MP4

- 5.22

- 5.68

- 6.69

HF

0.00

0.00

0.00

MP2

- 2.61

- 2.81

- 3.20

MP4

- 2.94

- 3.20

- 3.73

- 34.8 c m - 1 near R = 4.0 .~. The experimental well depths of Buck et al. are Vfa~e=-52.1 cm-~ at R = 3.4 A, Vedge = --53.2 c m - l at R = 3.6 ,~, and Vvertex=-33.1 cm-~ at R = 4 . 1 A in which the edge configuration has the most favorable interaction [21]. Yin and Mackerell [19] obtained the well depths Vface = - 3 1 . 8 cmol at R = 3 . 6 A, Vedge = - 2 2 . 4 cm -l at R = 3.9 A, Vvertex= - 2 0 . 2 cm -1 at R = 4.2 ,~ at the MP3/6-31G(3d,3p) level with BSSE correction. At the MP3/6-311 + + G(3d,3p) level without BSSE correction,° they obtained Vfa~e= - 1 5 4 . 7 cm~ l at R = 3 . 3 A, V~dge= - l l l . 3 cm~ 1 at R = 3.5 A, Vvert~x= - 137.5 cm -1 at R = 3.9 A. From the above comparison, we conclude that our calculated potential is the most reliable for the N e C H 4 system; while the interaction energies are over underestimated at the MP3/6-31G(3d,3p) level with BSSE correction, they are much overestimated at the MP3/6-311 + + G(3d,3p) level without BSSE correction.

D. Gao et al. / Chemical Physics Letters 277 (1997) 483-489

4. Conclusions We have presented an ab initio calculation of the potential energy surfaces for the H e - C H 4 and N e CH 4 systems at the MP4 level using an efficient basis set containing bond functions. The potential energy surface of H e - C H 4 is characterized by a global minimum corresponding to a symmetrical configuration (face configuration) of the system and two saddle points with strong angular-radial coupling. It can be anticipated that our calculated potential surfaces of H e - C H 4 and N e - C H 4 will be quite informative for spectroscopists in experimental studies.

Acknowledgements This work was supported in part by NATO under Grant CRG 941209. We thank Professor U. Buck for helpful communication.

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