The VMFCI method: A flexible tool for solving the molecular vibration problem

Abstract

The present article introduces a general variational scheme to find approximate solutions of the spectral problem for the molecular vibration Hamiltonian. It is called the "vibrational mean field configuration interaction" (VMFCI) method, and consists in performing vibrational configuration interactions (VCI) for selected modes in the mean field of the others. The same partition of modes can be iterated until self-consistency, generalizing the vibrational self-consistent field (VSCF) method. As in contracted-mode methods, a hierarchy of partitions can be built to ultimately contract all the modes together. So, the VMFCI method extends the traditional variational approaches and can be included in existing vibrational codes based on the latter approaches. The flexibility and efficiency of this new method are demonstrated on several molecules of atmospheric interest.

Ab initio calculation of the rotational spectrum of methane vibrational ground state

“…The eigenvalue problem for each H 0 has been solved by performing vibrational mean field configuration interaction (VMFCI) calculations as implemented in the computer code CONVIV. 13 The method has been described in details in Refs. 1, 8, 9, 59, and 60.…”

“…For this purpose, we have calculated our own high level dipole moment surface (DMS) and have determined the vibrational wave functions by means of the variational vibrational mean field configuration interaction approach. 13 It is important to have such a theoretical reference, independent of any experimental number, because the value of the effective dipole moment of methane is still a debated question. Recent experimental studies have obtained quite different values.…”

From atoms to biomolecules: a fruitful perspective

“…Extensive calculations were also performed on methane [25][26][27][28][29], with the goal to predict the rotation-vibration spectrum which is of particular interest for evaluating methane abundance in planetary atmospheres. Our most recent results use an accurate potential energy surface calculated at the CCSD(T)/ACV5Z level of theory [119] and a dipole moment surface that we calculated at the IC-MRCI/ ACV5Z level.…”

“…The vibrational Schrödinger equation has been solved variationally, and perturbation theory was used to calculate the rotational spectrum of the ground vibrational state [29]. Note that the variational method developed for solving the vibrational problem, the vibrational mean field configuration interaction method (VMFCI) [26,27], is directly inspired from electronic ab initio SCF/CI theory. The rotational spectrum calculated in that way is in very good agreement with the one recently recorded at the SOLEIL synchrotron [14] and with the atmospheric HI-TRAN database [142].…”

An improved third order dipole moment surface for methane

“…Eigenstates converged to within the cm −1 accuracy (or better), up to, say, the tetradecad for a given PES have been obtained by means of various forms of configuration interactions (CI) [30][31][32][33][34] (earlier references can be found therein). We have used the vibrational mean field configuration interaction (VMFCI) method [35][36][37][38][39] to solve methane vibrational, stationary Schrödinger equation.…”