No abstract

Conspectus Chemistry on the scale of molecular clusters may be dramatically different from that in the macroscopic bulk. Greater understanding of chemistry in this size regime could greatly influence fields such as materials science and atmospheric and environmental chemistry. Recent advances in experimental techniques and computational resources have led to accurate investigations of the energies and spectral properties of weakly bonded molecular clusters. These have enabled researchers to learn how the physicochemical properties evolve from individual molecules to bulk materials and to understand the growth patterns of clusters. Experimental techniques such as infrared, microwave, and photoelectron spectroscopy are the most popular and powerful tools for probing molecular clusters. In general, these experimental techniques do not directly reveal the atomistic details of the clusters but provide data from which the structural details need to be unearthed. Furthermore, the resolution of the spectral properties of energetically close cluster conformers can be prohibitively difficult. Thus, these investigations of molecular aggregates require a combination of experiments and theory. On the theoretical front, researchers have been actively engaged in quantum chemical ab initio calculations as well as simulation-based studies for the last few decades. To obtain reliable results, there is a need to use correlated methods such as Møller-Plesset second order method, coupled cluster theory, or dispersion corrected density functional theory. However, due to nonlinear scaling of these methods, optimizing the geometry of large clusters still remains a formidable quantum chemistry challenge. Fragment-based methods, such as divide-and-conquer, molecular tailoring approach (MTA), fragment molecular orbitals, and generalized energy-based fragmentation approach, provide alternatives for overcoming the scaling problem for spatially extended molecular systems. Within MTA, a large system is broken down into two or more subsystems that can be readily treated computationally. Finally, the properties of the large system are obtained by patching the corresponding properties of all the subsystems. Due to these approximations, the resulting MTA-based energies carry some error in comparison with calculations based on the full system. An approach for correcting these errors has been attempted by grafting the error at a lower basis set onto a higher basis set. Furthermore, investigating the growth patterns and nucleation processes in clusters is necessary for understanding the structural transitions and the phenomena of magic numbers in cluster chemistry. Therefore, systematic building-up or the introduction of stochastics for generating molecular assemblies is the most crucial step for studying large clusters. In this Account, we discuss the working principle of MTA for probing molecular clusters at ab initio level followed by a brief summary of an automated and electrostatics-guided algorithm for building molecular assemblies. The molecul...

High level ab initio investigations on molecular clusters are generally restricted to those of small size essentially due to the nonlinear scaling of corresponding computational cost. Molecular tailoring approach (MTA) is a fragmentation-based method, which offers an economical and efficient route for studying larger clusters. However, due to its approximate nature, the MTA-energies carry some errors vis-à-vis their full calculation counterparts. These errors in the MTA-energies are reduced by grafting the correction at a lower basis set (e.g., 6-31+G(d)) onto a higher basis set (e.g., aug-cc-pvdz or aug-cc-pvtz) calculation at MP2 level of theory. Further, better estimates of energies are obtained by making use of many-body interaction analysis. For this purpose, R-goodness (Rg) parameters for the three- and four-body interactions in a fragmentation scheme are proposed. The procedure employing grafting and many-body analysis has been tested out on molecular clusters of water, benzene, acetylene and carbon dioxide. It is found that for the fragmentation scheme having higher three- and four-body Rg-values, the errors in MTA-grafted energies are reduced typically to ~0.2 mH at MP2 level calculation. Coupled with the advantage in terms of computational resources and CPU time, the present method opens a possibility of accurate treatment of large molecular clusters.

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.

Clusters of CO(2) are a subject of detailed experimental as well as theoretical investigations due to their interesting applications. In the present article, CO(2) clusters (n = 6 to 13) are studied at the MP2 level of theory. The clusters are grown using a cluster building algorithm developed by our group and the larger ones are optimized at the MP2/aug-cc-pVDZ level by employing a Molecular Tailoring Approach (MTA). Vibrational spectra of these clusters are also calculated at this level of theory within MTA. The computed vibrational frequencies for an asymmetric C-O stretch generally exhibit a blue shift with increasing cluster size. This observation is in agreement with the experimental results. MTA-single point energies for each cluster size at the MP2/aug-cc-pVTZ level are also calculated for estimating the interaction energies at the complete basis set limit.