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The computation of the nuclear quantum dynamics of molecules is challenging,requiring both accuracy and efficiency to be applicable to systems of interest.Recently, theories have been developed for employing time-dependent basisfunctions (denoted modals) with vibrational coupled cluster theory (TDMVCC).The TDMVCC method was introduced along with a pilot implementation, whichillustrated good accuracy in benchmark computations. In this paper we report anefficient implementation of TDMVCC covering the case where the wave functionand Hamiltonian contain up to two-mode couplings. After a careful regrouping ofterms, the wave function can be propagated with a cubic computational scalingwith respect to the number of degrees of freedom. We discuss the use of arestricted set of active one-mode basis functions for each mode, as well as twointeresting limits: i) the use of a full active basis where the variationalmodal determination amounts essentially to the variational determination of atime-dependent reference state for the cluster expansion; and ii) the use of asingle function as active basis for some degrees of freedom. The latter casedefines a hybrid TDMVCC/TDH approach which can obtain an even lowercomputational scaling. The resulting computational scaling for hybrid and fullTDMVCC[2] are illustrated for polyaromatic hydrocarbons (PAHs) with up to 264modes. Finally, computations on the internal vibrational redistribution onbenzoic acid (39 modes) are used to show the faster convergence of TDMVCC/TDHhybrid computations towards TDMVCC compared to simple neglect of some degreesof freedom.

Efficient time-dependent vibrational coupled cluster computations with  time-dependent basis sets at the two-mode coupling level: full and hybrid  TDMVCC[2]