Author ORCID Identifier

0009 0008 7375 9508

Author Linked-In Account

www.linkedin.com/in/nivedhitha-p-9a499b403

Biosketch

I completed my undergraduate and postgraduate studies in Chemistry at Gobi Arts and Science College, Erode. I later joined SASTRA Deemed University as a Research Scholar under the supervision of Dr. Subrata Banik. My doctoral research primarily focused on the development and implementation of the vibrational coupled cluster method for large molecular systems. During my Ph.D., I published four papers in peer-reviewed journals as part of my thesis work. In addition, I was actively involved in collaborative research, resulting in four more publications. Two of my thesis chapters were also presented at national and international conferences. I successfully completed my viva voce on April 6, 2026. I am currently working as a Postdoctoral Fellow at IIT Bombay, Mumbai.

Date of Award

6-4-2026

Document Type

Thesis

School

School of Chemical & Biotechnology

Programme

Ph.D.-Doctoral of Philosophy

First Advisor

Dr.Subrata Banik

Keywords

Vibrational Coupled Cluster Method, Vibrational Spectrum, Potential Energy Surface, Polycyclic Aromatic Hydrocarbons, DFT Functionals and Basis Set

Abstract

The Watson Hamiltonian is a widely used model for describing anharmonic vibrational motion in polyatomic molecules. It expresses the kinetic energy in separable dimensionless normal coordinates and represents the potential energy as a Taylor series expansion around the equilibrium geometry, usually up to the quartic approximation. Several computational methods have been developed to solve the corresponding vibrational Schrödinger equation for predicting infrared (IR), Raman, and vibrational circular dichroism (VCD) spectra. These include the Vibrational Self-Consistent Field (VSCF) method, Vibrational Configuration Interaction (VCI) and its approximations, and Vibrational Second-Order Perturbation Theory (VPT2), each providing different balances between computational cost and accuracy.

The Coupled Cluster Method (CCM) is considered one of the most accurate approaches for solving quantum many-body problems and has been successfully extended to vibrational systems through the Vibrational Coupled Cluster Method (VCCM). In this study, the bosonic representation of VCCM is adopted, where the effective harmonic oscillator (EHO) ground state serves as the reference vacuum, and harmonic oscillator ladder operators are used as excitation operators. Excited vibrational states are described using Coupled Cluster Linear Response Theory (CCLRT), which requires constructing and diagonalizing an effective Hamiltonian. However, this computationally demanding step restricts the application of conventional VCCM to relatively small molecular systems.

To address this limitation, this thesis develops an efficient Reduced Vibrational Coupled Cluster Method (RVCCM) based on a divide-and-conquer strategy. In this approach, vibrational modes are divided into target and bath sets. The target modes are treated using the VCCM method, while the bath modes are approximated using the Effective Harmonic Oscillator (EHO) model. This strategy significantly reduces computational cost while maintaining high accuracy. The developed method is applied to investigate complex high-energy vibrational regions, particularly the CH, NH, and OH stretching bands, where strong vibrational resonances and congested spectral features make conventional methods such as VSCF and VPT2 less effective.

The proposed RVCCM is further applied to study the Raman spectrum of furfural, with particular emphasis on the C=O stretching region, which is dominated by several multiquanta vibrational resonances. The influence of dynamic polarizability on Raman spectra at excitation wavelengths of 325.0 nm and 632.8 nm is also examined. The simulated Raman spectra obtained using RVCCM show excellent agreement with experimental observations, demonstrating the capability of the method to accurately describe highly anharmonic vibrational phenomena and complex spectral structures.

The accuracy of anharmonic vibrational calculations strongly depends on the quality of the Quartic Potential Energy Surface (QPES), which requires higher-order derivatives of the electronic energy. Since generating QPES becomes computationally expensive for larger molecules, this work performs a comprehensive benchmark of various Density Functional Theory (DFT) functionals for molecules containing more than 18 atoms. In addition, the influence of different atomic basis sets on anharmonic vibrational calculations is systematically investigated to identify computationally efficient combinations that provide reliable accuracy for QPES generation.

Finally, the benchmarked DFT functional and optimized basis set are employed to construct accurate QPES for RVCCM simulations of infrared spectra, with special emphasis on the CH stretching region. Detailed analyses of the corresponding vibrational wavefunctions provide valuable insights into anharmonic vibrational excitations and band structures. Overall, the proposed RVCCM framework offers an efficient and accurate computational approach for simulating vibrational spectra of medium- and large-sized polyatomic molecules, extending the applicability of vibrational coupled cluster theory to systems that were previously beyond practical computational limits.

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