Welcome to Homepage of Debashree Ghosh
- Development of QM/MM methods based on effective fragment potential
- Application of Canonical Transformation theory
- Cumulant approximated n-electron valence perturbation theory
- Density Matrix Renormalization Group self-consistent field
- Light harvesting processes
Effective fragment potential
Chemical and biological processes generally occur in solution and therefore, many degrees of freedom need to be considered. This is not possible by the use of high level correlated quantum mechanical methods. Thus, to understand the important solvent effects in such processes there is need for some QM/MM method. I am interested in developing QM/MM method formed by combination of effective fragment potential with high level correlated quantum mechanical methods like equuation of motion coupled cluster. This kind of method can be used to understand:
- π interaction in stacked nucleic base pairs
- Solvent effect in DNA.
Canonical transformation theory
Canonical transformation theory is a dynamic correlation method that can be used for multi-reference systems. In case of single reference problems, coupled cluster forms a very successful class of methods. Canonical transformation theory uses a similar exponential ansatz for the incorporation of dynamic correlation in multi-reference systems. There can be interesting application of the methods in case of:
- Conjugated polymers
- Biologically relevant carotenoids and retinals.
Cumulant approximated n-electron valence perturbation theory
N-electron valence perturbation theory is a recently developed multi-reference perturbation theory. However, it is plagued by high computational scaling due to the need for higher body reduced density matrices in the valence or active space. Cumulant approximation of higher body reduced density matrices by the lower body ones can be used to reduce the computational scaling thereby increasing the range of applicability of the multi-reference perturbation theory. This approximate form of the multi-reference perturbation theory can be combined with density matrix renormalization group to be applied to:
- Metal dimer bonding curves
- Conjugated polyenes
- Carotenoid molecules.
Density matrix renormalization group self-sonsistent field
Complete active space self-consistent field (CASSCF) has been used as a standard method to handle the multi-configurational nature of many systems. However, CASSCF spans over the complete space of configurations possible in the active space and therefore exponential in scaling with the size of the active space. Many of the interesting chemical and biological systems need a large active or valence space for a correct qualitative picture. Density matrix renormalization group has emerged as a powerful method for handling the multi-reference nature in the active space with polynomial scaling. Thus, a method that couples CASSCF with DMRG in the active space gives rise to a multi-reference static correlation method that can handle much larger active spaces than possible by traditional methods such as CASSCF. This can be applied to:
- Carotenoids
- Transition metal complexes.
Light harvesting processes
The traditionally accepted mechanism for the process of light harvesting was through the 3 states of the carotenoid molecule (S0 (ground state), S1 (optically inactive first excited state) and S2 (optically active state)). However over the last decade a few intermediate dark states have been observed experimentally. This has lead to the suspicion that the light harvesting process may not be as believed traditionally. Theoreticians have tried to elucidate the mechanism by various calculations. However due to the large valence space and the multireference nature of the problem arising from extensive conjugation many of the methods are not well suited. There is need for multi-reference methods for both static and dynamic correlation that can handle large active or valence spaces. DMRG and methods that rely on DMRG can be used to treat such problems to accurately find the number and energy of the intermediate dark states in the carotenoid molecules and therefore, shed light to the mechanism of light harvesting.
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