Low energy isomers of (H2O)25 from a hierarchical method based on Monte Carlo temperature basin paving and molecular tailoring approaches benchmarked by MP2 calculations

Abstract

We report new global minimum candidate structures for the (H2O)25 cluster that are lower in energy than the ones reported previously and correspond to hydrogen bonded networks with 42 hydrogen bonds and an interior, fully coordinated water molecule. These were obtained as a result of a hierarchical approach based on initial Monte Carlo Temperature Basin Paving sampling of the cluster's Potential Energy Surface with the Effective Fragment Potential, subsequent geometry optimization using the Molecular Tailoring Approach with the fragments treated at the second order Møller-Plesset (MP2) perturbation (MTA-MP2) and final refinement of the entire cluster at the MP2 level of theory. The MTA-MP2 optimized cluster geometries, constructed from the fragments, were found to be within <0.5 kcal/mol from the minimum geometries obtained from the MP2 optimization of the entire (H2O)25 cluster. In addition, the grafting of the MTA-MP2 energies yields electronic energies that are within <0.3 kcal/mol from the MP2 energies of the entire cluster while preserving their energy rank order. Finally, the MTA-MP2 approach was found to reproduce the MP2 harmonic vibrational frequencies, constructed from the fragments, quite accurately when compared to the MP2 ones of the entire cluster in both the HOH bending and the OH stretching regions of the spectra.

Toward an Accurate and Inexpensive Estimation of CCSD(T)/CBS Binding Energies of Large Water Clusters

“…This section presents the computational details for estimating accurate binding energies (BEs) of water clusters at the gold standard CCSD­(T) − level of theory employing MTA, − assisted by the grafting procedure. − The latter is a method for accurate and inexpensive estimation of CCSD­(T) level BEs of large-water clusters at the CBS limit …”

“…The sign (−1) K −1 is determined by the number K of the fragments included at each summation. The details of the fragmentation and working principle of MTA-based calculations can be found from the earlier research publications of our group. − …”

Molecular Tailoring Approach for the Estimation of Intramolecular Hydrogen Bond Energy

“…Recently, we have widened the application of the MTA-based method for the estimation of individual O-H•••O HB energy and their cooperativity contribution in water clusters [88]. Many works reported in the literature focus on the global minimum structure of water clusters of variable sizes [73,75,76,[100][101][102][103]. In spite of a large number of studies, the nature of water clusters at the molecular level is not fully understood.…”

Making and breaking of small water clusters: A combined quantum chemical and molecular dynamics approach

“…Also, as per expectations, the six‐membered cyclic ring is more stable than the chain form, which is evident from the S HB , C HB plots and the characteristic time constant values. Studies on larger water n ‐mers ( n > 6) are available in the literature . Although the focus of this work is small water clusters and using the small water cluster dynamics to understand the bulk liquid water properties, we have done some single‐point frequency calculations for [(H 2 O) n , n = 8, 10, 12, 14, and 16] both in the gas phase as well as with implicit solvation and calculated the corresponding HBSBIC from the obtained Hessians.…”