Ring-Polymer Instanton Tunneling Splittings of Tropolone and Isotopomers using a Δ-Machine Learned CCSD(T) Potential: Theory and Experiment Shake Hands.

Tropolone, a 15-atom cyclic molecule, has received much interest both experimentally and theoretically due to its H-transfer tunneling dynamics. An accurate theoretical description is challenging owing to the need to develop a high-level potential energy surface (PES) and then to simulate quantum-mechanical tunneling on this PES in full dimensionality. Here, we tackle both aspects of this challenge and make detailed comparisons with experiments for numerous isotopomers.

Constructing Potential Energy Surface with Correlated Theory for Dipeptides Using Molecular Tailoring Approach.

We demonstrate a cost-effective alternative employing the fragment-based molecular tailoring approach (MTA) for building the potential energy surface (PES) for two dipeptides viz. alanine-alanine and alanine-proline employing correlated theory, with augmented Dunning basis sets. About 1369 geometries are generated for each test dipeptide by systematically varying the dihedral angles and .

Hydration shell model for expeditious and reliable individual hydrogen bond energies in large water clusters.

Recently, we have developed and tested a method, based on the molecular tailoring approach (MTA-based) to directly estimate the individual hydrogen bond (HB) energies in molecular clusters. Application of this MTA-based method to large molecular clusters is prohibitively difficult due to the evaluation of the energy of large-sized fragments. We propose here a smaller model system called the shell model, to overcome this difficulty.

Enabling Rapid and Accurate Construction of CCSD(T)-Level Potential Energy Surface of Large Molecules Using Molecular Tailoring Approach.

The construction of a potential energy surface (PES) of even a medium-sized molecule employing correlated theory, such as CCSD(T), is arduous due to the high computational cost involved. The present study reports the possibility of efficiently constructing such a PES of molecules containing up to 15 atoms and 550 basis functions by employing the fragment-based molecular tailoring approach (MTA) on off-the-shelf hardware. The MTA energies at the CCSD(T)/aug-cc-pVTZ level for several geometries of three test molecules, viz.

Molecular Tailoring Approach for Estimating Individual Intermolecular Interaction Energies in Benzene Clusters.

There is no general method available for the estimation of individual intermolecular interaction energies in weakly bound molecular clusters, and such studies are limited only to the dimer. Recently, we proposed a molecular tailoring approach-based method for the estimation of individual O-H···O hydrogen bond energies in water clusters. In the present work, we extend the applicability of this method for estimating the individual intermolecular interaction energies in benzene clusters, which are expected to be small.

Encaged molecules in external electric fields: A molecular "tug-of-war".

Response of polar molecules CH3OH and H2O2 and a non-polar molecule, CO2, as "guests" encapsulated in the dodecahedral water cage (H2O)20 "host," to an external, perturbative electric field is investigated theoretically. We employ the hybrid density-functionals M06-2X and ωB97X-D incorporating the effects of damped dispersion, in conjunction with the maug-cc-pVTZ basis set, amenable for a hydrogen bonding description. While the host cluster (cage) tends to confine the embedded guest molecule through cooperative hydrogen bonding, the applied electric field tends to rupture the cluster-composite by stretching it; these two competitive effects leading to a molecular "tug-of-war.

CH3OH⋯(H2O)n [n = 1-4] clusters in external electric fields.

For hydrogen-bonded neutral molecular clusters, response to an externally applied electric field can critically affect molecular cooperativity. In this light, response of dilute methanol-water admixtures to an external, perturbative electric field is studied at the simplest molecular level in the cluster configurations CH3OH⋯(H2O)n with "n" chosen to range from 1 to 4, employing the M06-2X hybrid functional in conjunction with the 6-311++G(2d,2p) basis set, well-suited for hydrogen bonding. Methanol is seen to favorably bond with the water molecules at its hydroxyl end up to certain characteristic maximum threshold field strengths beyond which the HOMO-LUMO energy-gap abruptly drops to zero culminating into a complete breakdown of the cluster.