Calculating rovibrationally excited states of H2D+ and HD2+ by combination of fixed node and multi-state rotational diffusion Monte Carlo

Abstract

In this work the efficacy of a combined approach for capturing rovibrational coupling is investigated. Specifically, the multi-state rotational DMC method is used in combination with fixed-node DMC in a study of the rotation vibration energy levels of H 2 D + and HD + 2 . Analysis of the results of these calculations shows very good agreement between the calculated energies and previously reported values. Where differences are found, they can be attributed to Coriolis couplings, which are large in these ions and which are not fully accounted for in this approach. 2 molecules that undergo large amplitude motions provides a significant chal-3 lenge to many theoretical approaches. When vibrations are small amplitude, 4 perturbation theory provides an attractive alternative, even for highly vibra-5 tionally excited states [1, 2]. On the other hand, molecules that undergo large 6 amplitude motions sample broad regions of the potential surface. As such, 7 expansions based on a zero-order reference structure become challenging, and 8 perturbatin theory can provide unphysical results. Significant advances have 9 been made for studies of triatomic molecules, including H + 3 and its deuter-10 ated analogues for which accurate rotation/vibration energy levels have been 11 reported [3, 4]. Carrington and co-workers have made a variety of advances 12 that allow for the evaluation of vibrational energy levels and wave functions 13 for larger systems [5, 6] including the highly fluxional CH + 5 [7]. Recently 14 Fabri et al reported energies for rotationally excited states of low-lying vi-15 brational levels of H + 5 [8]. The multi-configurational time-dependent Hartree 16 approach (MCTDH) has also shown significant promise for studies of this 17 class of molecular systems [9, 10]. Wodraszka and Manthe reported results 18 of MCTDH calculations of energies of CH + 5 with J ≤ 2 [11] A challenge that 19comes in applying these approaches comes from the fact that the convergence 20 properties can be very sensitive to the choice of coordinates. 21Over the past several years, several groups have explored approaches based 22 on diffusion Monte Carlo (DMC) [12, 13] for evaluating either rotational or 23 vibrational excited states of molecules that undergo large amplitude vibra-24 tional motions even in their ground states [14, 15, 16, 17]. Systems that we 25