Research Projects

Recent advances in experimental techniques have led to accurate exploration of the energies and spectral properties. However, the assignment of particular molecular conformation to an experimentally observed spectrum becomes difficult for large systems as the experimental techniques do not directly reveal the atomistic details but provide data from which the structural details need to be unearthed. The quantum chemical (QC) vibrational analysis, in this context is invaluable for locating and identifying the conformers on potential energy surfaces (PES). Harmonic approximation is the simplest method but offers limited accuracy. Whereas, anharmonic calculations interpret experimental data more reliably and enable precise predictions of vibrational transitions. Many theoretical methods viz. vibrational self-consistent field/configuration interaction (VSCF/VCI), second-order vibrational perturbation theory (VPT2) and diffusion Monte Carlo method etc. have been applied for accurate prediction of anharmonic spectra. The anharmonic treatment deals with the construction of an adequate PES and no exact analytical solution is available. Reports on anharmonic treatments are very rare. Due to the inherent non-linear scaling of QC methods, ab initio-level investigation is a non-trivial task. It is indeed desirable to look for alternative solutions. There have been attempts for efficient computation of vibrational spectra, mainly employing fragmentation-based methods. However, most of the methods are restricted to harmonic spectra. In this proposal, we are developing an efficient procedure for computing the anharmonic vibrational spectra within molecular tailoring approach (MTA), a fragmentation method mainly meant for calculations with computer economy. The current project deals with the development and the implementation of VSCF/VCI and VPT2 methods to MTA for enabling cost-effective anharmonic spectra calculation. In line with the parallelization of fragmentation calculations, the current project opens up a new possibility to achieve precise vibrational features by pushing the boundaries of QC methods. The development will be incorporated to MTASpec software. Geometry optimization followed by anharmonic spectra calculations for a variety of systems comprising of clusters, biomolecules such as peptides, proteins etc. will be performed within MTA using ab initio methods. The calculated spectra will be benchmarked either with the full conventional ones or observed spectra. Effect of solvents on the chemistry of molecules will also be studied through their anharmonic spectra analysis. The present study suggests that the anharmonic spectra calculations of relatively large systems using MTA and HPC facility can be clearly envisioned in the near future. In collaboration with Prof. T. Nakajima and Dr. S. Khire, we will port our codes on Fugaku supercomputer at the RIKEN, Kobe, Japan, which extends the utility of the method to thousands of processors/nodes.