A BSSE-corrected CASSCF/NEVPT2 procedure. An application to weakly bonded OH..π heterodimer complexes

Abstract

In this work, a stable NEVPT2-based computational procedure was developed, capable of studying weakly bonded OH..π heterodimer complexes. The procedure was applied to the evaluation of the weak OH..π intermolecular interaction energy of the ethene–water C₂H₄–H₂O complex, as a model case. The counterpoise method of Boys and Bernardi was used with the strongly contracted (SC) and partially contracted (PC) variants of the NEVPT2 method and the energetic results were benchmarked against CCSD(T) calculations. In particular, for the first time a computational methodology is proposed for the appropriate specification of the active space in order to study weakly bonded OH..π heterodimer complexes, using the super-molecular approach. The treatment of weakly bonded OH..π and van der Waals complexes using CASSCF wavefunctions with second-order perturbation theory seems to render trustable and accurate results. Also, the present methodology suggests an efficient way for the specification of the ‘ghost’ basis functions in the multiconfigurational heterodimer case. The Basis Set Superposition Error (BSSE) was eliminated in both CASSCF(2,2) and CASSCF(10,7) selected case studies of the C₂H₄–H₂O dimer. The behavior of BSSE lowering as the basis set increases was verified. The computational procedure which was developed in this paper can be easily adapted to the multiconfigurational NEVPT2 treatment of a large variety of weakly bonded heterodimers. Finally, the procedure was successfully tested in the benzene-water heterodimer.

Keywords:

• CASSCF
• NEVPT2
• BSSE
• Weakly bonded complexes
• OH..intermolecular interactions

Acknowledgements

The authors would like to thank Professor Kenneth Ruud for his insightful comments and suggestions concerning this work. Also, the computer time provided by the Computer Center and the Computational Materials Science Laboratory of the University of Ioannina in Greece is gratefully acknowledged.