ИСТИНА

The traditional matrix variational method for calculating the vibrational spectra of molecules has limitations due to the exponential growth of the number of basis functions with increasing number of atoms. One of the alternatives to this method is the n-th order canonical Van Vleck operator perturbation theory (CVPTn) [1]. Using halogenated methane derivatives (CH2X2), it was shown that the fundamental frequencies obtained using CVPT4 correlate well with the results obtained by the variational method [2]. An interesting alternative to these methods is the vibrational self-consistent field method (VSCF) [3], which is often used in combination with further refinement by solving an additional variational problem in the basis of virtual functions (VSCF/CI). In this work, we compared the ab initio fundamental vibrational frequencies of formaldehyde (H2C=O) calculated by the Van Vleck perturbation theory (CVPT2/4) and the VSCF/CI method using the the hybrid MP2/aug- cc-pVTZ & CCSD(T)/cc-pVQZ (harmonic part) quantum-mechanical quartic and sextic force fields in normal coordinates (Table 1). It is shown that when using a sextic force field the VSCF/CI provides a high level of numerical coincidence with results obtained by the CVPT4 method. [1] doi:10.1021/jp211400w, S. V. Krasnoshchekov, E. V. Isayeva and N. F. Stepanov, J. Phys. Chem. A, 116, 3691–3709 (2012). [2] doi:10.1063/1.5020295, S. V. Krasnoshchekov, R. S. Schutski, N. C. Craig, M. Sibaev and D. L. Crittenden, J. Chem. Phys., 148, 084102 (2018). [3] doi:10.1002/9780470141199.ch4, R. B. Gerber and M. A. Ratner, Int. Rev. Phys. Chem., 70, 97–132 (1988).