The VCI-P code: an iterative variation–perturbation scheme for efficient computations of anharmonic vibrational levels and IR intensities of polyatomic molecules

Modelization of vibrational spectra beyond the harmonic approximation from an iterative variation–perturbation scheme: the four conformers of the glycolaldehyde

“…V (q1,…, qM) is a polynomial expansion of the PES in terms of normal coordinates q i truncated to the fourth order The last term represents the major component of the rotational contribution to the anharmonicity in which B a is the rotational constant of the system with respect to the Cartesian axis a, and f a ij is the Coriolis constant that couples q i and q j through the rotation about the a axis. The anharmonic vibrational treatment is based on an iterative variational-perturbational scheme implemented in the VCI-P code [16]. This code computes the anharmonic wavenumbers and the anharmonic IR intensities of the fundamental transitions, overtones and combination bands which are IR (or Raman) actives through strong anharmonic couplings (Fermi or Darling Dennisson resonances).…”

“…For a better accuracy, the correlation between modes is commonly treated by (i) the vibrational Moller-Plesset perturbation theory (VMP) [10] that is computationally cheap but overestimates the strong anharmonic complings, (ii) a vibrational configuration interaction (VCI) [8] that treats properly the strongest interactions but proves much more time consuming, (iii) the vibrational mean field configuration interaction (VMFCI) [11] that plays on the partitioning of the vibrational modes and encompasses both VSCF and VCI as particular case and (iv) the vibrational coupled cluster (VCC) [12] level of theory. Several strategies to strongly reduce the computational cost of such treatment were proposed in the literature [13][14][15][16] (see Ref. [16] for a more detailed description).…”

“…Several strategies to strongly reduce the computational cost of such treatment were proposed in the literature [13][14][15][16] (see Ref. [16] for a more detailed description). A possible way which is used in the present study is to take advantage of both perturbative and variational approaches [17].…”

“…A possible way which is used in the present study is to take advantage of both perturbative and variational approaches [17]. This variational-perturbational scheme in its most advanced version has been implemented in the VCI-P code [16] that uses small VCI matrices to treat the strongest interactions while the myriads of the weakest interactions are treated perturbationally (see the section computational details and method). Such a recipe was used to investigate the vibrational properties of the four conformers of glycolaldehyde (CH 2 OHCHO).…”

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

“…As an example Table 3 shows computed infrared intensities for selected vibrational overtones and combination bands of formaldehyde obtained at different levels of theory. 69,98 The intensities obtained from RHF dipole surfaces clearly deviate from those determined at the QCISD(T) level, but may serve as a good guess obtained from negligible additional computational costs.…”