01428nas a2200205   4500008004100000245008900041210006900130260000800199300001000207490000700217520073100224100002600955700002600981700002101007700002301028700002101051700001701072700001901089856011401108       2008                        eng d00aIntroduction to the density matrix renormalization group ansatz in quantum chemistry0 aIntroduction to the density matrix renormalization group ansatz   cJAN  a49-650 v183 aThe Density Matrix Renormalisation Group (DMRG) is an electronic structure method that has recently been applied to ab-initio quantum chemistry. Even at this early stage, it has enabled the solution of many problems that would previously have been intractable with any other method, in particular, multireference problems with very large active spaces. Historically, the DMRG was not originally formulated from a wavefunction perspective, but rather in a Renormalisation Group (RG) language. However, it is now realised that a wavefunction view of the DMRG provides a more convenient, and in some cases more powerful, paradigm. Here we provide an expository introduction to the DMRG ansatz in the context of quantum chemistry.1 aChan, Garnet, Kin-Lic1 aDorando, Jonathan, J.1 aGhosh, Debashree1 aHachmann, Johannes1 aNeuscamman, Eric1 aWang, Haitao1 aYanai, Takeshi  uhttps://library.ncl.res.in/content/introduction-density-matrix-renormalization-group-ansatz-quantum-chemistry02001nas a2200157   4500008004100000022001400041245007500055210006900130260007100199300001400270490000800284520140900292100001501701700002101716856010601737       2014                        eng d  a1089-563900aIonization-induced tautomerization in cytosine and effect of solvation0 aIonizationinduced tautomerization in cytosine and effect of solv  a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcJUL  a5323-53320 v1183 a<p>The recent observation of excitation-induced tautomerization in gas-phase cytosine motivated us to investigate the possibility of facile tautomerization in ionized cytosine and the effect of solvation on the tautomerization barriers. The tautomerization mechanisms were characterized at the density functional theory (DFT)omega B97X-D and coupled-cluster singles and doubles (CCSD) levels of theory. Vertical and adiabatic ionization energies (VIEs and AIEs, respectively) of the tautomers of cytosine and the microsolvated species were calculated with the equation-of-motion ionization-potential coupled-cluster (EOM-IP-CCSD) method. We observed that, in microsolvated cytosine, the solvatochromic shifts of the VIEs can be both blue- and red-shifted depending on the tautomers. This is explained by the analysis of the charge dipole interactions between the cytosine and water molecules. We noticed that, upon ionization, gas-phase tautomerization barriers are reduced by 0-4 kcal/mol, whereas microsolvated (with one water) tautomerization barriers are reduced by 4-5 kcal/mol. We also investigated the tautomerization process in solvation using a continuum model with one active water molecule in the quantum mechanical region. We noticed that, even though bulk solvation has a significant effect on ionization energies, its effect on the ionization-induced tautomerization barrier is minimal.</p>1 aDas, Tamal1 aGhosh, Debashree  uhttps://library.ncl.res.in/content/ionization-induced-tautomerization-cytosine-and-effect-solvation-001565nas a2200145   4500008004100000245011300041210006900154260000800223300001400231490000700245520100700252100001601259700002101275856012301296       2017                        eng d00aInteraction energy driven biased sampling technique: a faster route to ionization spectra in condensed phase0 aInteraction energy driven biased sampling technique a faster rou  cOCT  a2248-22570 v383 aWe introduce a computationally efficient approach for calculating spectroscopic properties, such as ionization energies (IEs) in the condensed phase. Discrete quantum mechanical/molecular mechanical (QM/MM) approaches for spectroscopic properties in a dynamic system, such as aqueous solution, need a large sample space to obtain converged estimates, especially for the cases where particle (electron) number is not conserved, such as IEs or electron affinities (EAs). We devise a biased sampling technique based on an approximate estimate of interaction energy between the solute and solvent, that accelerates the convergence and therefore, reduces the computational cost significantly. The approximate interaction energy also provides a good measure of the spectral width of the chromophores in the condensed phase. This technique has been tested and benchmarked for (i) phenol, (ii) HBDI anion (hydroxybenzylidene dimethyl imidazolinone), and (iii) thymine in water. (c) 2017 Wiley Periodicals, Inc.1 aBose, Samik1 aGhosh, Debashree  uhttps://library.ncl.res.in/content/interaction-energy-driven-biased-sampling-technique-faster-route-ionization-spectra