Advances in vibrational configuration interaction theory ‐ part 2: Fast screening of the correlation space

Abstract

For larger molecules, the computational demands of configuration selective vibrational configuration interaction theory (cs-VCI) are usually dominated by the configuration selection process, which commonly is based on second order vibrational Møller-Plesset perturbation (VMP2) theory. Here we present two techniques, which lead to substantial accelerations of such calculations while retaining the desired high accuracy of the final results. The first one introduces the concept of configuration classes, which allows for a highly efficient exploitation of the analogs of the Slater-Condon rules in vibrational structure calculations with large correlation spaces. The second approach uses a VMP2 like vector for augmenting the targeted vibrational wavefunction within the selection of configurations and thus avoids any intermediate diagonalization steps. The underlying theory is outlined and benchmark calculations are provided for highly correlated vibrational states of several molecules.

On the vibrations of formic acid predicted from first principles

“…However, the straightforward construction of the untruncated VCI matrix and computation of its eigenvectors is prohibitively expensive and in practice approximations are introduced. 33,34 By partitioning the modes into a set of active and a set of bath modes Mizukami and Tew 35 have introduced the vibrational active space self-consistent field theory (VASSCF), vibrational active space configuration interaction (VASCI) and vibrational active space second order perturbation theory (VASPT2) methods. These methods employ a product of a CI type ansatz for the active modes and a simple product ansatz for the bath modes.…”

Quantum-chemical calculation of two-dimensional infrared spectra using localized-mode VSCF/VCI

“…[58][59][60][61] While the calculation of anharmonic vibrational energy levels is less established, most impor-tantly because it is associated with a higher computational effort, tremendous progress has been made in the past decades. [62][63][64][65][66][67][68][69][70] Most importantly, efficient approximate schemes for the inclusion of anharmonicities in theoretical vibrational spectroscopy have become available that are applicable to medium-seized molecules, including polypeptides. [71][72][73][74] Here, we extend one such approach, the localized-mode vibrational self-consistent field (L-VSCF) method in combination the with localized-mode vibrational configuration interaction (L-VCI) method, 75,76 for the calculation of 2D-IR spectra.…”

Calculation of absolute Raman scattering cross‐sections using vibrational self‐consistent field/vibrational configuration interaction wave functions

“…(i) Efficient generation of PESs, [26][27][28] (ii) Use of optimized and localized vibrational coordinates, [29][30][31][32] (iii) Contracting and Pruning basis functions, [33][34][35][36][37] (iv) The Canonical Polyadic and Tensor Train tensor decompositions, [38][39][40][41][42][43] and (v) Approaches to select vibrational configuration interaction (VCI) spaces efficiently. [44][45][46] A branch of variational and mixed approaches to solving the vibrational equation is grounded on the vibrational self-consistent field (VSCF) method, 47,48 which is usually combined with other methods, including (i) VCI using VSCF states, herein represented by the VSCF/ VCI acronym, 49,50 (ii) vibrational Møller-Plesset (VSCF/VMP), 51,52 (iii) vibrational coupled-cluster (VSCF/VCC), 53,54 among others. 55 Concerning Raman and ROA spectroscopies, Danĕ cek et al 56 calculated those spectra for alanine and proline zwitterions using: (i) VSCF wave functions, (ii) the RSPT2 method, applied to the Harmonic (Harm/PT2) and VSCF solutions and (iii) Harm/VCI wave functions.…”