ETKİN BAĞLANMIŞ KÜME YÖNTEMLERİNİN PROGRAMLANMASI VE KİMYASAL UYGULAMALARI

Abstract

In modern quantum chemistry, Coupled-Cluster (CC) based methods and programs are known to provide highly reliable results. These methods, which have attracted much attention in the literature, are being developed together with integral approximation techniques to be applied to larger-scale molecular systems and to overcome the computational difficulties arising from two-electron integrals (TEIs). Programs that store two-electron integrals in core memory are considerably faster than disk-based algorithms. Density fitting (DF) and Cholesky decomposition (CD) are recently popular approximation techniques. In this study, to apply the Orbital Optimized Coupled-Cluster Doubles (OCCD) method to large-scale molecular systems, DF and CD techniques, which are the most modern integral tensor approximation techniques in recent times, are applied. Thus, DF-CCSD, DF-OCCD, and CD-OCCD methods are developed for the first time with UHF reference. The developed methods are efficiently coded in C++ language and added to Psi4 (https://psicode.org) and MacroQC (https://www.macroqc.hacettepe.edu.tr) programs and presented to the users.

Furthermore, Equation-of-Motion (EOM) based methods have been developed, which are known to provide highly accurate results in the calculation of excited state energies. The equations of motion developed for the standard and OO-based CC and MP (Møller-Plesset, MP) theories are efficiently coded in C++ and added to MacroQC software. Many of the DF-EOM-CC, DF-EOM-OO-CC, DF-EOM-MP and DF-EOM-OO-MP methods developed in this context are developed for the first time. The developed methods are named as DF-EOM-CCSD, DF/CD-EOM-CCSD, DF-EOM-CCD, DF-EOM-OCCD, DF-EOM-MP2, DF-EOM-OMP2, DF-EOM-MP3, DF-EOM-OMP3, DF-EOM-MP2.5 and DF-EOM-OMP2.5. In addition, to evaluate the accuracy and efficiency of the methods developed here, several chemical systems with challenging electronic structures are considered. In the light of these methods, we attempt to elucidate the nature of the chemical structures considered. In this context, problematic molecules such as various open and closed shell molecular systems, organic chromophores, radical structures formed from polyaromatic hydrocarbons, and water clusters are investigated. This thesis brings important innovations to the field of theoretical chemistry by programming methods that are lacking in the literature and presenting chemical application studies in popular fields.