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Accurate determination of molecular spectra and structure: Interplay between experiment and theory (in honor of Peter Botschwina)
Chemical Physics 346 (2008) vii–viii
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Preface
Accurate determination of molecular spectra and structure: Interplay between experiment and theory (in honor of Peter Botschwina)
This issue of Chemical Physics is dedicated to Professor Peter Botschwina on the occasion of his 60th birthday. It is intended to honor his academic and scientific achievements over the period of several decades of his research in the field of ab initio quantum mechanical calculations. Professor Botschwina is well known in the spectroscopic community for his careful theoretical treatments with best available electronic-structure methods. Already his early work which he carried out in the group of W. Meyer (Kaiserslautern) dealt with force fields, vibrational frequencies and structures of small molecules. When he moved to Göttingen, he continued to work in the field of theoretical molecular spectroscopy and theoretical reaction dynamics. Current emphasis is on topics such as calculations of accurate potential-energy surfaces and their analytical representation, calculations of molecular properties such as vibrational frequencies, rotational constants, electric dipole moments, intensities, ionisation potentials and electronic excitation energies. Also in the focus of his interest are calculations of barrier heights for 0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2008.04.001
chemical reactions, lifetimes of reactive resonances and photodissociation dynamics. His publications concern mainly reactive species – radicals or ions – of importance in combustion processes, atmospheric chemistry and interstellar cloud chemistry. Other topics are hydrogen-bonded molecular clusters and intermediates of SN2 chemical reactions. Professor Botschwina has been particularly interested in linear neutral and charged carbon chains, including also heteroatoms, which play an important role in astrophysics. In many cases his results proved to be very useful for assignments of structures and rotational–vibrational transitions. For instance, the experimental information alone is mostly not sufficient to obtain equilibrium re structures of polyatomic molecules. The combination with theoretical calculations can provide more precise structural spectroscopic constants. Due to his profound experience in this important area of research he was solicited by many spectroscopists in Europe and Overseas to collaborate in the interpretation of high-resolution spectra. Other contributions of Peter Botschwina
viii
Editorial / Chemical Physics 346 (2008) vii–viii
concern negatively and positively charged complexes and anion photoelectron spectroscopy. He has published several invited review articles about accurate equilibrium structures, interstellar molecules, proton-bound cluster ions and small anions. His outstanding scientific reputation has been honored by the full membership in the Academy of Sciences in Göttingen. The numerous contributions in this issue which come from friends, colleagues, as well as present and former co-workers of Professor Botschwina document his international recognition in various areas of theoretical chemistry and spectroscopy and highlight the interplay between experiment and theory in spectroscopy. The first group of articles focuses on very accurate treatments for small molecules, including radicals and charged species, for ground as well as electronically excited states. The methods applied for the electronic-structure problem are coupled-cluster (CC) approaches wherever possible, with up to full triple excitations, or methods of the multi-reference configuration interaction type (CASSCF/MRCI). Very large basis sets are used, or explicitly correlated schemes are applied which will extend the frontier of accurate ab initio methods in the future. The properties calculated are thermochemical data, equilibrium re structures, dipole moments, as well as ionization potentials and electron affinities. Also reported are a variety of spectroscopic data such as harmonic wavenumbers, vibration–rotation coupling constants, etc. Often large parts of the potential-energy surfaces (PES) as well as dipole moment functions and transition moments are calculated, and rovibrational spectra, IR and UV radiative transition probabilities and photoelectron (PE) spectra are determined. Where necessary, corrections are made for scalar-relativistic effects, spin–orbit coupling, or for core correlation contributions. Impressive accuracies are reached, indeed, in the 1 kJ/mol range for atomization energies and few cm1 for fundamental vibrational frequencies. This is a good basis for a meaningful interaction with experiment, and a fruitful synergy is actually documented in nearly all of the papers, concerning, e.g., the interpretation of vibrational, PE, or chemiluminescence spectra. Nuclear dynamics on larger parts of the electronic PES and inclusion of non-adiabatic effects can be handled very accurately for small molecules, as shown by various papers in the second part of this volume. For Hþ 2 , e.g., non-adiabatic effects can be accounted for by using a geometry-dependent effective nuclear mass on a single potential surface, and accuracies of 102 cm1 can be reached for vibration–rotational levels. Special methods can be applied for the nuclear dynamics of pseudo-rotating molecules or for treating the Renner–Teller effect, where large-amplitude motions of the nuclei are taken into account for the calculation of low-lying vibronic excitations. The accurate determination of excited-state potential surfaces allows for a reliable quantum-dynamics treatment of photodissociation and for a better understanding of experimental data concerning cross sections, internal energy distributions, etc. The third part of this volume is devoted to the determination of rotational and vibrational spectra of medium-size and large molecules and complexes. The articles feature a close interaction be-
tween theoretical and experimental work, sometimes in a joint effort from both sides. Prerequisites for the theoretical determination of rovibrational levels are accurate potential-energy surfaces (and dipole moment, polarizability surfaces) which are often obtained at the coupled-cluster (CCSD(T)) or MP2 level for molecules with four to several tens of atoms. Of great potential for the future are vibrational CI methods for state-selective calculation of fundamental vibrational modes, overtones and low-lying combination levels. Experimental methods include Fourier transform spectroscopy for infrared and microwave measurements as well as infrared photodissociation spectroscopy, yielding a variety of molecular constants like centrifugal distortion, anharmonicity, and l-type resonance parameters, and also allowing for discussion of hyperfine structure, hydrogen-bond effects on O–H overtone intensities, etc. Very often, information from theoretical work is helpful here, e.g., structural information in the case of ionic complexes, or vibrational corrections to various molecular properties. An upcoming field, where calculations may become very useful, is the calculation of Raman optical activity spectra. Theoretical models may also help for the determination of (ro-)vibrational densities of states which are important for reaction kinetics. The systems addressed in the present volume range from NHþ 3 over molecules with Cn chains þ to Al ðCH4 Þn clusters and oxo-anions in water. The treatments are not always restricted to rovibrational spectra, but also include molecular dynamics (MD) simulations with the discussion of ligand exchange. Structure determination for complexes of weakly bound molecules, i.e. systems with hydrogen or van der Waals bonds, is at the focus of the last part of the collection of papers in this volume. The applied methods range from ab initio CCSD(T), QCISD(T), and MP2 approaches over DFT schemes with/without empirical dispersion corrections to approaches using fully empirical potentials. Questions on how to efficiently perform basis-set extrapolation and apply counterpoise corrections are an issue here. Development of efficient ab initio methods is still an active research field; examples are local correlation methods, or range-hybrid coupling schemes between ab initio and DFT approaches. This is even more important for excitation energies (e.g., intermolecular charge tranfer) where standard time-dependent DFT methods often yield unsatisfactory results. Finally, structure optimization is often not a trivial task for such systems, and there is still much to be learnt about the relation between intermolecular potentials and cluster structures. We are grateful to all colleagues who contributed articles to this issue, and we thank Prof. Ludwig Hofacker without whose support the present special issue of Chemical Physics would not have been possible. We hope that Peter Botschwina will enjoy this tribute to his work and that he will continue his excellent and important research for many years to come. The scientific community will gratefully acknowledge many more contributions to theoretical spectroscopy and dynamics from his laboratory. H.-J. Werner P. Rosmus H. Stoll
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Preface
Accurate determination of molecular spectra and structure: Interplay between experiment and theory (in honor of Peter Botschwina)
This issue of Chemical Physics is dedicated to Professor Peter Botschwina on the occasion of his 60th birthday. It is intended to honor his academic and scientific achievements over the period of several decades of his research in the field of ab initio quantum mechanical calculations. Professor Botschwina is well known in the spectroscopic community for his careful theoretical treatments with best available electronic-structure methods. Already his early work which he carried out in the group of W. Meyer (Kaiserslautern) dealt with force fields, vibrational frequencies and structures of small molecules. When he moved to Göttingen, he continued to work in the field of theoretical molecular spectroscopy and theoretical reaction dynamics. Current emphasis is on topics such as calculations of accurate potential-energy surfaces and their analytical representation, calculations of molecular properties such as vibrational frequencies, rotational constants, electric dipole moments, intensities, ionisation potentials and electronic excitation energies. Also in the focus of his interest are calculations of barrier heights for 0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2008.04.001
chemical reactions, lifetimes of reactive resonances and photodissociation dynamics. His publications concern mainly reactive species – radicals or ions – of importance in combustion processes, atmospheric chemistry and interstellar cloud chemistry. Other topics are hydrogen-bonded molecular clusters and intermediates of SN2 chemical reactions. Professor Botschwina has been particularly interested in linear neutral and charged carbon chains, including also heteroatoms, which play an important role in astrophysics. In many cases his results proved to be very useful for assignments of structures and rotational–vibrational transitions. For instance, the experimental information alone is mostly not sufficient to obtain equilibrium re structures of polyatomic molecules. The combination with theoretical calculations can provide more precise structural spectroscopic constants. Due to his profound experience in this important area of research he was solicited by many spectroscopists in Europe and Overseas to collaborate in the interpretation of high-resolution spectra. Other contributions of Peter Botschwina
viii
Editorial / Chemical Physics 346 (2008) vii–viii
concern negatively and positively charged complexes and anion photoelectron spectroscopy. He has published several invited review articles about accurate equilibrium structures, interstellar molecules, proton-bound cluster ions and small anions. His outstanding scientific reputation has been honored by the full membership in the Academy of Sciences in Göttingen. The numerous contributions in this issue which come from friends, colleagues, as well as present and former co-workers of Professor Botschwina document his international recognition in various areas of theoretical chemistry and spectroscopy and highlight the interplay between experiment and theory in spectroscopy. The first group of articles focuses on very accurate treatments for small molecules, including radicals and charged species, for ground as well as electronically excited states. The methods applied for the electronic-structure problem are coupled-cluster (CC) approaches wherever possible, with up to full triple excitations, or methods of the multi-reference configuration interaction type (CASSCF/MRCI). Very large basis sets are used, or explicitly correlated schemes are applied which will extend the frontier of accurate ab initio methods in the future. The properties calculated are thermochemical data, equilibrium re structures, dipole moments, as well as ionization potentials and electron affinities. Also reported are a variety of spectroscopic data such as harmonic wavenumbers, vibration–rotation coupling constants, etc. Often large parts of the potential-energy surfaces (PES) as well as dipole moment functions and transition moments are calculated, and rovibrational spectra, IR and UV radiative transition probabilities and photoelectron (PE) spectra are determined. Where necessary, corrections are made for scalar-relativistic effects, spin–orbit coupling, or for core correlation contributions. Impressive accuracies are reached, indeed, in the 1 kJ/mol range for atomization energies and few cm1 for fundamental vibrational frequencies. This is a good basis for a meaningful interaction with experiment, and a fruitful synergy is actually documented in nearly all of the papers, concerning, e.g., the interpretation of vibrational, PE, or chemiluminescence spectra. Nuclear dynamics on larger parts of the electronic PES and inclusion of non-adiabatic effects can be handled very accurately for small molecules, as shown by various papers in the second part of this volume. For Hþ 2 , e.g., non-adiabatic effects can be accounted for by using a geometry-dependent effective nuclear mass on a single potential surface, and accuracies of 102 cm1 can be reached for vibration–rotational levels. Special methods can be applied for the nuclear dynamics of pseudo-rotating molecules or for treating the Renner–Teller effect, where large-amplitude motions of the nuclei are taken into account for the calculation of low-lying vibronic excitations. The accurate determination of excited-state potential surfaces allows for a reliable quantum-dynamics treatment of photodissociation and for a better understanding of experimental data concerning cross sections, internal energy distributions, etc. The third part of this volume is devoted to the determination of rotational and vibrational spectra of medium-size and large molecules and complexes. The articles feature a close interaction be-
tween theoretical and experimental work, sometimes in a joint effort from both sides. Prerequisites for the theoretical determination of rovibrational levels are accurate potential-energy surfaces (and dipole moment, polarizability surfaces) which are often obtained at the coupled-cluster (CCSD(T)) or MP2 level for molecules with four to several tens of atoms. Of great potential for the future are vibrational CI methods for state-selective calculation of fundamental vibrational modes, overtones and low-lying combination levels. Experimental methods include Fourier transform spectroscopy for infrared and microwave measurements as well as infrared photodissociation spectroscopy, yielding a variety of molecular constants like centrifugal distortion, anharmonicity, and l-type resonance parameters, and also allowing for discussion of hyperfine structure, hydrogen-bond effects on O–H overtone intensities, etc. Very often, information from theoretical work is helpful here, e.g., structural information in the case of ionic complexes, or vibrational corrections to various molecular properties. An upcoming field, where calculations may become very useful, is the calculation of Raman optical activity spectra. Theoretical models may also help for the determination of (ro-)vibrational densities of states which are important for reaction kinetics. The systems addressed in the present volume range from NHþ 3 over molecules with Cn chains þ to Al ðCH4 Þn clusters and oxo-anions in water. The treatments are not always restricted to rovibrational spectra, but also include molecular dynamics (MD) simulations with the discussion of ligand exchange. Structure determination for complexes of weakly bound molecules, i.e. systems with hydrogen or van der Waals bonds, is at the focus of the last part of the collection of papers in this volume. The applied methods range from ab initio CCSD(T), QCISD(T), and MP2 approaches over DFT schemes with/without empirical dispersion corrections to approaches using fully empirical potentials. Questions on how to efficiently perform basis-set extrapolation and apply counterpoise corrections are an issue here. Development of efficient ab initio methods is still an active research field; examples are local correlation methods, or range-hybrid coupling schemes between ab initio and DFT approaches. This is even more important for excitation energies (e.g., intermolecular charge tranfer) where standard time-dependent DFT methods often yield unsatisfactory results. Finally, structure optimization is often not a trivial task for such systems, and there is still much to be learnt about the relation between intermolecular potentials and cluster structures. We are grateful to all colleagues who contributed articles to this issue, and we thank Prof. Ludwig Hofacker without whose support the present special issue of Chemical Physics would not have been possible. We hope that Peter Botschwina will enjoy this tribute to his work and that he will continue his excellent and important research for many years to come. The scientific community will gratefully acknowledge many more contributions to theoretical spectroscopy and dynamics from his laboratory. H.-J. Werner P. Rosmus H. Stoll