Quantum Monte Carlo simulations of selected ammonia clusters (n = 2–5): Isotope effects on the ground state of typical hydrogen bonded systems

Abstract

Variational Monte Carlo, diffusion Monte Carlo, and stereographic projection path integral simulations are performed on eight selected species from the (NH(3))(n), (ND(3))(n), (NH(2)D)(n), and (NH(3))(n-1)(ND(3)) clusters. Each monomer is treated as a rigid body with the rotation spaces mapped by the stereographic projection coordinates. We compare the energy obtained from path integral simulations at several low temperatures with those obtained by diffusion Monte Carlo, for two dimers, and we find that at 4 K, the fully deuterated dimer energy is in excellent agreement with the ground state energy of the same. The ground state wavefunction for the (NH(3))(2-5) clusters is predominantly localized in the global minimum of the potential energy. In all simulations of mixed isotopic substitutions, we find that the heavier isotope is almost exclusively the participant in the hydrogen bond.

Determining the dissociation threshold of ammonia trimers from action spectroscopy of small clusters

“…We report the theoretical values as the energy required to remove a single monomer unit from the trimer, using the calculated binding energies and zero-point energies from the calculations as needed. Two estimates by Mella and co-workers using MP2 calculations with a large basis set 8 and using quantum Monte Carlo techniques 7 give values of 1590 cm −1 and 1416 cm −1 , respectively. The calculation by Slipchenko et al 9 gives a higher value of 1794 cm −1 .…”

“…For example, they potentially make the individual hydrogen bonds in an oligomer stronger than the single hydrogen bond in the dimer. There is a large set of calculations on the binding energy and structure of the ammonia dimer, [1][2][3][4][5][6][7][8][9][10][11] but relatively few of them include zero-point effects that allow comparison with experimental dissociation energies. [7][8][9][10] There are fewer ab initio calculations on the structure of the trimer, [7][8][9][10][11] four of which include zero-point energy terms and predict the energy, E trimer , required to remove one NH 3 unit from the trimer, [7][8][9][10] (NH 3 ) 3 → (NH 3 ) 2 + NH 3 .…”

From the (NH3)2–5 clusters to liquid ammonia: Molecular dynamics simulations using the NVE and NpT ensembles

“…Curotto et al [53] determine that ZPE amounts to approximately 42% of the binding energy, which for 3.037 kcal mol -1 represents 1.275 kcal mol -1 . This means that our model gives an estimation of the dissociation energy, D 0 , equal to 1.762 kcal mol -1 , also in good agreement with the experimental results of 1.890 kcal mol -1 (661 cm -1 ) [51] and 1.621 kcal mol -1 (567 cm -1 ) [54].…”

On the convergence of diffusion Monte Carlo in non-Euclidean spaces. I. Free diffusion

“…[5][6][7][8][9] Most recently, we have used it to study the ground state of ethane and propane in an external field, 5 to perform simulations of the hydrogen molecule on ammonia clusters, 6 and to study oxygen in helium clusters. 7 These demanding applications of DMC have required a number of enhancements to the basic algorithm.…”