Jonathon K. Gregory

Calculations of the tunneling splittings in water dimer and trimer using diffusion Monte Carlo

The diffusion Monte Carlo (DMC) method is used to calculate rovibrational bound states of the water dimer and trimer. The rigid body form of DMC is employed, together with correlated sampling of energy differences between states of different symmetry. This allows calculation of the tunneling splittings in (H2O)2 and (H2O)3. The results for (H2O)2 are in quite good agreement with those obtained using a basis set method, and also agree well with experiment. In addition, we have made predictions for similar splittings in (D2O)3 and several water dimer isotopomers. In all the calculations, we have used the potential energy surface due to Millot and Stone which is known to give quite good agreement with experiment for the tunneling splittings in (H2O)2.

Three‐body effects on molecular properties in the water trimer

We report an application of diffusion Monte Carlo to investigate the importance of three‐body forces on the properties of the water trimer. The potential energy surface used is due to Millot and Stone and is based on intermolecular perturbation theory to which three‐body induction and dispersion energies are added. The effects of the three‐body forces are considered by comparison with the same potential containing only pairwise water interactions. We have calculated minimum energy structures, vibrationally averaged structures, zero‐point energies, rotational constants, cluster dissociation energies, and tunneling splittings, with and without the three‐body forces. The values obtained for the vibrationally averaged rotational constants with the three‐body potential are fairly close to the experimental values. Whereas the rotational constants are shown to have a significant dependence, the tunneling splittings are changed little by the three‐body forces. Based on the calculated difference in anharmonic zero...

Tunneling dynamics in water tetramer and pentamer

We present a theoretical study of the structures, energetics and tunneling dynamics of the cyclic water tetramer and pentamer, both of which have recently been observed experimentally. The very good agreement between vibrationally averaged rotational constants from diffusion quantum Monte Carlo calculations and experimental values confirms that the structures are cyclic. We calculate a splitting for a tunneling motion that is a likely cause for doublets observed experimentally in the far infrared spectrum of (D2O)4. Predictions of tunneling splittings yet to be observed in the water pentamer are made.

A comparison of conventional and rigid body diffusion Monte Carlo techniques. Application to water dimer

Abstract Diffusion Monte Carlo calculations on the ground state of the water dimer are reported. Two versions of the method have been used. The first is the conventional method with importance sampling which simulates all vibrations in the dimer and the second is one due to Buch which treats the water monomers as rigid bodies and therefore removes their vibrational motion. We find good agreement between the results of the two approaches in terms of their calculations of zero-point energies, geometries and rotational constants.

Quantum simulation of the benzene-water complex

A theoretical investigation is presented of the weakly bound complex formed between benzene and water. Diffusion quantum Monte Carlo methods are used to describe the nuclear motion plus two potentials which give quite good agreement with DZP/MP2 ab initio calculations, and simulations were performed for four isotopomers of C6H6H⋅⋅⋅H2O. Although the minimum energy structure can be considered to have only a single hydrogen bond, vibrational averaging renders the hydrogens indistinguishable, a prediction in agreement with the experimental observation that the complex is a symmetric top. The results include zero-point energies, vibrationally averaged structures, rotational constants and wavefunctions. By calculating transition states and rearrangement mechanisms, it is possible to characterize the tunnelling dynamics and calculate the associated tunnelling splittings.

The C6H6-(H2O)2 complex: theoretical predictions of the structure, energetics, and tunneling dynamics

A detailed theoretical study of the C6H6–(H2O)2 complex is presented. We characterize the structure and energy by means of various potentials and correlated ab initio calculations. The potential surface is extremely flat but the structures obtained with the empirical potentials and ab initio optimizations agree fairly well. Transition states and corresponding reaction paths are calculated for four possible degenerate rearrangements. The splittings for these mechanisms are calculated from quantum simulations with the diffusion Monte Carlo (DMC) approach. We predict that two splittings should be observable in the spectrum. The DMC calculations also allow prediction of vibrationally averaged structures, bond energies, and rotational constants.

Diffusion Monte Carlo studies of isotope-substituted water trimers

Abstract We report the ground-state properties of several partially deuterated water trimers calculated with the rigid-body diffusion Monte Carlo method. Rotational constants are compared with recent experiments and good agreement is found. Tunneling splittings for the hydrogen flip in (HDO) 3 are predicted. New results are predicted for the experimentally unexamined mixed trimers.

Reaction path zero-point energy from diffusion Monte Carlo calculations

A general diffusion quantum Monte Carlo method is described for accurately calculating the zero‐point energy of the vibrations orthogonal to a reaction path in a polyatomic system. The method fully takes into account anharmonic and mode–mode coupling effects. The algorithm is applied to the OH+H2→H2O+H reaction and the results are compared with a more approximate calculation. The technique will have many useful applications to kinetic and spectroscopic problems involving polyatomic molecules.

Quantum simulation of weakly bound complexes using direct ab initio energy points

Abstract We describe the use of a quantum diffusion Monte Carlo algorithm coupled directly to an ab initio program which supplies the (MP2) electronic energy for the quantum simulation. This method does not require a fit to a multidimensional potential energy surface. We demonstrate the application of this method to (HF) 2 for which we have calculated the fully anharmonic zero-point energy, the ground state rotational constants, ro-vibrational wavefunctions and the tunnelling splitting. The method we describe is computationally demanding, but general, and will be particularly attractive for use on parallel computers.

A method to calculate vibrational frequency shifts in heteroclusters: application to N2+–Hen

A method is proposed whereby vibrational frequency shifts can be calculated in weakly bound heteroclusters and is applied to the ionic N2+–Hen clusters. We use diffusion Monte Carlo to simulate the 3n degrees of freedom of the N2+–Hen interactions while the N–N vibrational motion is solved by an adiabatic method. In addition, we report minimum energy structures, vibrationally averaged structures and binding energies for N2+–Hen clusters for n= 1–12.