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Computational simulation of excited states is one of the most active areas of research in modern day quantum chemistry. Popularly used density functional theory (DFT) based methods show poor performance for excited states, especially for charge transfer or Rydberg excited states. Wave-function based methods can provide the desired accuracy but cannot be used beyond small molecules due to the prohibitive computational costs. Recently, local correlation approaches have become popular for reducing the computational scaling of wave-function based methods. However, local orbital based methods have often struggled with characterizing excited states, as excited states are often non-local. We have devised a hybrid strategy (STEOM)1 by combining local correlation approach with the use of successive similarity transformations to arrive at an N5 scaling method. Further decreases in storage requirements have been achieved by using semi-numerical techniques2 for integral evaluation. The resulting method can calculate the ionization potential, electron affinity, and excitation energies of molecules containing hundreds of atoms, with accuracy comparable to state of the art coupled cluster methods.3,4,5,6
To investigate the effect of the surrounding environment on the excitation energy, we are developing a local density-embedding scheme for excited states. The new scheme allows one to perform accurate wave-function based excited state calculations on the chromophore embedded in a Hartree-Fock density of the surrounding environment. The local density-embedding scheme has the unique advantages over the existing multi-level quantum chemical approaches that it does not require explicit cutting of bonds. These new methods have been made available as a part of the popular quantum chemistry package ORCA.
References
[1] A. K. Dutta, F. Neese and R. Izsák J. Chem. Phys. 034102, 145 (2016).
[2] A. K. Dutta, F. Neese and R. Izsák J. Chem. Phys. 034102, 144 (2016).
[3] A. K. Dutta, M. Nooijen, F. Neese and R. Izsák. J. Chem. Theor. Comput. 72, 14 (2017)
[4] A. K. Dutta, M. Nooijen, F. Neese and R. Izsák J. Chem. Phys. 074103, 146 (2017).
[5] A. K. Dutta, F. Neese and R. Izsák J. Chem. Phys. 214111, 146 (2017).
[6] A. K. Dutta, M. Saitow, C. Riplinger. F. Neese and R. Izsák J. Chem. Phys. 244101, 148 (2018). |