Full-dimensional quantum calculations of ground-state tunneling splitting of malonaldehyde using an accurate ab initio potential energy surface

Abstract

Quantum calculations of the ground vibrational state tunneling splitting of H-atom and D-atom transfer in malonaldehyde are performed on a full-dimensional ab initio potential energy surface (PES). The PES is a fit to 11 147 near basis-set-limit frozen-core CCSD(T) electronic energies. This surface properly describes the invariance of the potential with respect to all permutations of identical atoms. The saddle-point barrier for the H-atom transfer on the PES is 4.1 kcalmol, in excellent agreement with the reported ab initio value. Model one-dimensional and "exact" full-dimensional calculations of the splitting for H- and D-atom transfer are done using this PES. The tunneling splittings in full dimensionality are calculated using the unbiased "fixed-node" diffusion Monte Carlo (DMC) method in Cartesian and saddle-point normal coordinates. The ground-state tunneling splitting is found to be 21.6 cm(-1) in Cartesian coordinates and 22.6 cm(-1) in normal coordinates, with an uncertainty of 2-3 cm(-1). This splitting is also calculated based on a model which makes use of the exact single-well zero-point energy (ZPE) obtained with the MULTIMODE code and DMC ZPE and this calculation gives a tunneling splitting of 21-22 cm(-1). The corresponding computed splittings for the D-atom transfer are 3.0, 3.1, and 2-3 cm(-1). These calculated tunneling splittings agree with each other to within less than the standard uncertainties obtained with the DMC method used, which are between 2 and 3 cm(-1), and agree well with the experimental values of 21.6 and 2.9 cm(-1) for the H and D transfer, respectively.

A generalized reactive force field for nonlinear hydrogen bonds: Hydrogen dynamics and transfer in malonaldehyde

“…A more recent study using an extensive number of points calculated at the CCSD͑T͒ level with extrapolation to the near-basis set limit reports a tunneling splitting of 21-22 cm −1 depending on how the coordinates are treated. 25 The barrier for H-atom transfer is found to be 4.1 kcal/mol, which is close to the value of 4.3 kcal/mol found on the present MMPT surface. Earlier work has found barriers of 3.6 kcal/mol and corresponding tunneling splittings of 13.9 cm −1 by employing MP2/ 6-31G͑d,p͒ for the PES and classical dynamics simulations from which tunneling splittings were estimated by using semiclassical theory.…”

“…The barrier of V 0 ͑s͒ obtained by CHARMM/MMPT optimizations is ⌬E b = 4.3 kcal/ mol, which agrees quite well with the above CCSD͑T͒/6-311+ +G ‫ءء‬ value at the MP2/6-311+ +G ‫ءء‬ geometries and also agrees with 4.1 kcal/mol reported by Bowman group from CCSD͑T͒/aug-cc-pV5Z PES. 25 Using the harmonic frequencies from the MMPT potential leads to a zero point energy corrected HBA barrier height …”

Ab initio potential energy surface of CH+2 and reaction dynamics of H + CH+

“…1 Sketch of coordinates used to describe the CH + 2 molecular system. The potential energy surface (PES) is constructed following Braams et al [16][17][18][19][20] and Murrell et al 21 . In the original ansatz, the potential energy surface is fitted in terms of polynomials of exponential functions of internuclear distances, y i j = exp (−r i j /λ), where λ is a scale parameter and i and j are nuclear indices.…”

Perspective: Ring-polymer instanton theory

“…82,89 This excellent result is due to a high level of accuracy not only of instanton theory but also of the potentialenergy surfaces fitted to high-level ab initio calculations. 90,91 Note that even small errors in the surface will affect the action, S, and thus lead to exponentially larger errors in the predicted splitting. The accuracy of the PES is therefore of the utmost importance.…”