Vibrational multiconfiguration self-consistent field theory: Implementation and test calculations

Abstract

A state-specific vibrational multiconfiguration self-consistent field (VMCSCF) approach based on a multimode expansion of the potential energy surface is presented for the accurate calculation of anharmonic vibrational spectra. As a special case of this general approach vibrational complete active space self-consistent field calculations will be discussed. The latter method shows better convergence than the general VMCSCF approach and must be considered the preferred choice within the multiconfigurational framework. Benchmark calculations are provided for a small set of test molecules.

MOVIPAC: Vibrational spectroscopy with a robust meta‐program for massively parallel standard and inverse calculations

“…[29,33,52] Subspace iteration methods for vibrational problems have been used before in the context of molecular mechanics applications, [53][54][55][56] where huge Hessians must be diagonalized and where subspace iteration techniques are a natural means to solve the diagonalization problem. In quantum chemistry, however, the Mode-Tracking idea established new principles [28,[57][58][59][60][61][62][63] : (1) isolated, structure-characteristic vibrations can be directly targeted, (2) vibrations that involve only a subset of atoms (e.g., in the case of adsorbates on surfaces [64] or in quantum mechanics (QM)/molecular mechanics (MM) partitionings [65] ) can be directly optimized, (3) low-dimensional Hessians also benefit if their entries are time-consuming to calculate. In our former work, [51] it was shown that the proper construction of a guess vector has an important influence on the convergence characteristics of the algorithm.…”

“…The subscript ' 'eq' ' indicates that the second derivative has to be taken at an equilibirum geometry, that is at a stationary point on the potential energy surface. More accurate methods for the variational calculation of molecular vibrations including anharmonic effects [2][3][4][5][6][7] are under constant development but can hardly be applied to truly large molecules containing on the order of 100 atoms and more. Hence, molecular dynamics approaches are often used as a means to include anharmonic effects on the motion of atomic nuclei [8,9] but suffer from the neglect of quantum effects on the nuclear motion.…”

Selected Aspects Concerning the Efficient Calculation of Vibrational Spectra beyond the Harmonic Approximation

“…This situation corresponds to static electron correlation effects as any kind of resonance cannot be described by the VSCF approach and thus the VSCF wave function is qualitatively incorrect. For that reason Culot and Lievin 89,90 developed the vibrational multiconfiguration self-consistent field approach, VMCSCF, which can be considered the static variant of the multi-configuration time-dependent Hartree method, MCTDH, of Meyer et al 91−93 Quite recently we presented a novel and efficient implementation of VMCSCF theory 34,35 based on Jacobi 2×2 rotations as originally proposed by Hinze for electronic wave functions.    ˆ, ...…”

“…For further details see Ref. 34. Consequently, these equations can be solved by consecutive Jacobi rotations for each mode (microiterations).…”

“…Acta 85 (2012) 379. obtained from multi-configuration self-consistent field theory, VMCSCF. [34][35][36] Moreover, we will discuss some aspects concerning the efficient calculation of integrals relevant for all vibration correlation methods. Some minor aspects concerning infrared intensities and the calculation of vibronic transitions will also be discussed.…”