Automatic Computation of Potential Energy Surfaces for MCTDH Quantum Dynamics using Reparametrized Semiempirical Hamiltonians
Résumé
A correct description of quantum effects is necessary if a physically correct molecular simulation is aimed for [1]. Despite the current boost in semiclassical and full quantum dynamical methods, the availability of a Potential Energy Surfaces (PES) is still a major bottleneck. For grid- based ones, the PES is represented globally, formally as a multidimensional tensor and, in the case of on-the-fly approaches, the PES is expressed in a local representation at every time-step. In the first category, the limitation lies on the possibility of fitting the PES to an appropriate functional form for a large number of degrees of freedom. Powerful and accurate as the existing methods are, a high degree of expertise is still required to master and apply these techniques, particularly when considering medium-large systems (>6D), thus preventing a wider-spread use. In the second case, the limiting factor is the number of electronic structure calls (energies, gradients, Hessians, properties, etc.) needed to perform the propagation. Consequently, on-the-fly approaches are constrained to modest levels of theory.
We present Specific Reaction Parameter Multigrid POTFIT (SRP-MGPF) [2], a method which constitutes a well-balanced solution to the aforementioned issues. SRP-MGPF allows, in a single fitting process, the generation of a chemically-accurate (<1 kcal/mol) global (molecular or intermolecular) PES at the cost of semiempirical potentials. SRP-MGPF relies on three steps: (i) a fully automated topographical characterisation of the PES in terms of all minima and transition states using the TSSCDS [3] or the vdW-TSSCDS [4] methods; (ii) a global reparametrization of a semiempirical Hamiltonian (SRP) using reference geometries derived from the set of stationary points [2]; and (iii) direct tensor-decomposition of the SRP PES into sum-of-products form with the MGPF algorithm [5]. To show the capabilities of our approach, we have compared Multiconfiguration Time-Dependent Hartree (MCTDH) [6] calculations of vibrational eigenstates and wavepacket propagations using a reference HONO (6D) PES of CCSD(T) quality [7] and our SRP-MGPF PES.
References
[1] F. Gatti (Ed.), Molecular Quantum Dynamics, Springer (Heidelberg), (2014).
[2] R. L. Panadés-Barrueta, E. Martínez-Núñez, D. Peláez, Front. Chem. (submitted)
[3] E. Martínez-Núñez, J. Comp. Chem. 36, 222 (2015).
[4] S. Kopec, E. Martínez-Núñez, J. Soto, D. Peláez, Int. J. Quantum Chem. (accepted).
[5] D. Peláez, H.-D. Meyer, J. Chem. Phys., 138, 014108 (2013).
[6] M. H. Beck, A. Jäckle, G. A. Worth, H.-D. Meyer, Phys. Rep., 324, 1 (2000).
[7] F. Richter, M. Hochlaf, P. Rosmus, F. Gatti, H.-D. Meyer, J. Chem. Phys., 120, 1306 (2004)