Ab initio calculation of anharmonic vibrational states of polyatomic systems: Electronic structure combined with vibrational self-consistent field

Abstract

An algorithm for first-principles calculation of vibrational spectroscopy of polyatomic molecules is proposed, which combines electronic ab initio codes with the vibrational self-consistent field (VSCF) method, and with a perturbation-theoretic extension of VSCF. The integrated method directly uses points on the potential energy surface, computed from the electronic ab initio code, in the VSCF part. No fitting of an analytic potential function is involved. A key element in the approach is the approximation that only interactions between pairs of normal modes are important, while interactions of triples or more can be neglected. This assumption was found to hold well in applications. The new algorithm was applied to the fundamental vibrational excitations of H2O, Cl−(H2O), and (H2O)2, using the Möller–Plesset method for the electronic structure. The vibrational frequencies found are in very good accord with experiments. Estimates suggest that this electronic ab initio/VSCF approach should be feasible, with reasonable computational resources, for all-mode calculations of vibrational energies and wave functions for systems of up to 10–15 atoms. The new method can be also very useful for testing the accuracy of electronic structure codes by comparing with experimental vibrational spectroscopy.

First-principles anharmonic quantum calculations for peptide spectroscopy: VSCF calculations and comparison with experiments

“…22,23,38 To bring in effect of non-separability, the vibrational levels can be further corrected by second-order perturbation theory (VSCF-PT2). 33,38 This method is typically more accurate than the basic VSCF approximation. Any perturbative methods may suffer from degeneracy issues which are discussed later.…”

“…In this contribution, we calculate the first-principles based anharmonic IR spectra of three protected dipeptides by means of quantum mechanically calculated PES using quantum VSCF-PT2 approximation 33 of a level that turns out to be sufficiently accurate for comparison with experiments. So far, VSCF calculations were carried out for peptides and a protein using empirical 34 and semi-empirical 25 potentials.…”

Dispersion corrected DFT approaches for anharmonic vibrational frequency calculations: nucleobases and their dimers

“…On the contrary, consistent procedures can be derived from QM computations of vibrational properties beyond the harmonic approximation. Recently, exact solutions for the treatment of few active modes to the vibrational problem for a generic system has been proposed 55 and effective schemes to compute vibrational frequencies within the second order vibrational perturbative (VPT2) [56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75] or vibrational self-consistent field (VSCF) based [76][77][78][79][80][81][82][83][84][85] approaches have been developed and implemented. In particular, a general VPT2 framework to compute thermodynamic properties, vibrational energies and transition intensities from the vibrational ground state to fundamentals, overtones and combination bands 66,67,[86][87][88] has been developed in our group.…”

Matrix effect on vibrational frequencies: Experiments and simulations for HCl and HNgCl (Ng = Kr and Xe)

“…This approach represents the first step in the VSCF algorithms. [51][52][53] It neglects the coupling between different vibrational modes and includes only the intrinsic anharmonicity of the normal modes.…”