Fragmentation methods offer an attractive alternative for treatment of large molecules and molecular clusters. However, balancing the accuracy and efficiency of these methods is a tight-rope-act. With this in view, we present an algorithm for automatic molecular fragmentation within Molecular Tailoring Approach (MTA) achieving this delicate balance. The automated code is tested out on a variety of molecules and clusters at the Hartree-Fock (HF)- and Møller-Plesset second order perturbation theory as well as density functional theory employing augmented Dunning basis sets. The results show remarkable accuracy and efficiency the respective full calculations. Thus the present work forms an important step toward the development of an MTA-based black box code for implementation of HF as well as correlated quantum chemical calculations on large molecular systems.

We propose a procedure, within the many-body analysis (MBA) framework, for an economic yet accurate estimation of the correlated method-based energies of large molecular clusters employing Dunning’s augmented basis sets. The basis of the procedure is to segregate the Hartree–Fock (E HF) and correlation energy (E C) estimations. E HF is found to differ by tens of millihartrees (mH) from its full-calculation (FC) counterpart on truncating the MBA expansion at the two-body (MBA-2) level. On the contrary, E C is estimated with smaller error on modest hardware with limited computation time at the (MBA-2) level. In view of this, we adopt a pragmatic method wherein the E HF (accurate to five decimal places) is taken from the FC, whereas E C is estimated at the MBA-2 level. This method is applied to a variety of medium to large molecular clusters at the MP2 level. Preliminary results at the CCSD­(T) level for (H2O)16 and (H2O)17 are also reported with tremendous savings in wall-clock time and resources. The typical errors in MP2 and CCSD­(T) energies per monomer are up to 0.1 and 0.2 mH, respectively. Thus the present method, balancing accuracy and computational economy, opens a way for estimating energies of large molecular clusters using correlated theories with large basis sets.

Fragmentation details for the W32-TIP4P-Ew isomer. The monomer indices only for the main fragments are listed below. Size indicates the number of water monomers in each main fragment.

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. The PES, of near CCSD(T)-quality, is obtained using a Δ-machine learning approach starting from a pre-existing low-level DFT PES and corrected by a small number of approximate CCSD(T) energies obtained using the fragmentation-based molecular tailoring approach. The resulting PES is benchmarked against DF-FNO-CCSD(T) and CCSD(T)-F12 calculations. Ring-polymer instanton calculations of the splittings, obtained with the Δ-corrected PES are in good agreement with previously reported experiments and a significant improvement over those obtained using the low-level DFT PES. The instanton path includes heavy-atom tunneling effects and cuts the corner, thereby avoiding passing through the conventional saddle-point transition state. This is in contradistinction with typical approaches based on the minimum-energy reaction path. Finally, the subtle changes in the splittings for some of the heavy-atom isotopomers seen experimentally are reproduced and explained.

A web-interface for geometry optimization of large molecules using a linear scaling method, i.e., cardinality guided molecular tailoring approach (CG-MTA), is presented. CG-MTA is a cut-and-stitch, fragmentation-based method developed in our laboratory, for linear scaling of conventional ab initio techniques. This interface provides limited access to CG-MTA-enabled GAMESS. It can be used to obtain fragmentation schemes for a given spatially extended molecule depending on the maximum allowed fragment size and minimum cut radius values provided by the user. Currently, we support submission of single point or geometry optimization jobs at Hartree-Fock and density functional theory levels of theory for systems containing between 80 to 200 first row atoms and comprising up to 1000 basis functions. The graphical user interface is built using HTML and Python at the back end. The back end farms out the jobs on an in-house Linux-based cluster running on Pentium-4 Class or higher machines using an @Home-based parallelization scheme