The accurate estimation of the ionization energies and understanding the nature of the ionized states of the nucleic acid bases (NABs) are crucial to the understanding of the DNA damage mechanism. The vertical ionization energy (VIE) of guanine is the lowest among the NABs and the ionization energies are strongly affected by the environment, such as solvation and characteristics of nearby NABs. Therefore, we investigate the sequence dependence of the VIEs of guanine in B-DNA. We use the equation of motion coupled cluster method for the estimation of ionization potential with single and double excitations (EOM-IP-CCSD) and density functional theory with dispersion corrected ωB97x-D for the estimation of VIEs. A significant amount of non-additivity or cooperativity, directly proportional to charge delocalization, is noticed in the change in VIE due to the interaction with the nearby NABs. While the change in VIE due to base pairing originates predominantly from charge-dipole interactions, stacking between base pairs is a more complicated balance of dispersion and charge-dipole interactions as well as stabilization due to the delocalization of the positive charge. The long range interactions are however dominated by 1/r(3) distance dependence which shows the major role played by charge-dipole interactions. The extent of localization of positive holes on guanine is also estimated for various sequences.

A stochastic configuration interaction method based on evolutionary algorithm is designed as an affordable approximation to full configuration interaction (FCI). The algorithm comprises of initiation, propagation and termination steps, where the propagation step is performed with cloning, mutation and cross-over, taking inspiration from genetic algorithm. We have tested its accuracy in 1D Hubbard problem and a molecular system (symmetric bond breaking of water molecule). We have tested two different fitness functions based on energy of the determinants and the CI coefficients of determinants. We find that the absolute value of CI coefficients is a more suitable fitness function when combined with a fixed selection scheme.Keywords: genetic algorithm, evolutionary algorithm, configuration interaction Majority of the electronic structure methods that have been developed and used over the last few decades starts with the independent orbital approximation, i.e. the assumption that a single Slater determinant is a qualitatively correct starting point for a calculation. This qualitatively correct reference is typically corrected for dynamic correlation with post Hartree Fock (HF) methods such as Moller-Plesset perturbation theory (MP2) or coupled cluster singles and doubles (CCSD). However, the assumption of a single Slater determinant as a reference is not qualitatively correct, especially in situations where there are significant orbital degeneracies or neardegeneracies, e.g., bond breaking or di-and tri-radicals. Such systems are referred to as strongly correlated systems and the electronic correlation in these systems are referred to as static correlation, as opposed to dynamic correlation. It is important to note that we are not differentiating between true correlation due to orbital degeneracies and that required to treat proper spin symmetry (non-dynamic and static correlations). 1 Full configuration interaction (FCI) is the most rigorous method to treat correlation, both static and dynamic. However, FCI involves exact diagonalization of the full Hamiltonian in its Hilbert space and is therefore, not affordable for reasonable system sizes and basis sets. 2 Therefore, approximate methods such as CASSCF 3 and RASSCF 4 etc have been developed where only a subset of the orbital space is treated exactly to reduce the computational cost. But these methods also involve an exact diagonalization, albeit over a smaller sub-space. On the other hand, density matrix renormalization group (DMRG) 5-9 have been developed to circumvent the exact diagonalization problem and therefore, the associated exponential scaling. While DMRG has been remarkably successful in the case of pseudo-linear systems, more general 2D and 3D systems are complicated due to problems in orbital ordering. 10,11 However, there have been a) http://academic.ncl.res.in/debashree.ghosh b) Electronic mail: debashree.ghosh@gmail.com developments towards using tensor networks to alleviate this problem. 12,13 Antisymmetrized geminal power (AGP) wavefunctions ha...

Ionization of nucleobases is affected by their biological environment, which includes both the effect of adjacent nucleotides as well as the presence of water around it. Guanine and its nucleotide have the lowest ionization potentials among the various DNA bases. Therefore, the threshold of ionization is dependent on that of guanine and its characterization is crucial to the prediction of interaction of light with DNA. We investigate the effect of solvation on the vertical ionization energies (VIEs) of guanine and its nucleotide. In this work, we have used hybrid quantum mechanics/molecular mechanics (QM/MM) approach with effective fragment potential as the MM method of choice and equation-of-motion coupled-cluster for ionization potential with singles and doubles (EOM-IP-CCSD) as the QM method. The performance of the hybrid scheme with respect to the full QM method shows an accuracy of ≤ 0.02-0.04 eV. The lowest few ionizations of the nucleotide are found to be from different parts of the moiety, that is, the nucleic acid base, phosphate, or sugar, and these ionization energies are very closely spaced giving rise to a very complicated spectrum. Furthermore, microsolvation has large effects on these ionizations and can lead to red or blue shift depending on the position of the water molecule. Even a single water molecule can change the order of ionized states in the nucleotide. The VIEs of the bulk solvated chromophores are predicted and compared to existing experimental spectra. The predominant role of polarization in the solvatochromic shift is noticed. © 2017 Wiley Periodicals, Inc.