The effect of polarizable environment on two-photon absorption cross sections characterized by the equation-of-motion coupled-cluster singles and doubles method combined with the effective fragment potential approach

Abstract

We report an extension of a hybrid polarizable embedding method incorporating solvent effects in the calculations of two-photon absorption (2PA) cross sections. We employ the equation-of-motion coupled-cluster singles and doubles method for excitation energies (EOM-EE-CCSD) for the quantum region and the effective fragment potential (EFP) method for the classical region. We also introduce a rigorous metric based on 2PA transition densities for assessing the domain of applicability of QM/MM (quantum mechanics/molecular mechanics) schemes for calculating 2PA cross sections. We investigate the impact of the environment on the 2PA cross sections of low-lying transitions in microhydrated clusters of para-nitroaniline, thymine, and the deprotonated anionic chromophore of photoactive yellow protein (PYPb). We assess the performance of EOM-EE-CCSD/EFP by comparing the 2PA cross sections against full QM calculations as well as against the non-polarizable QM/MM electrostatic embedding approach. We demonstrate that the performance of QM/EFP improves when few explicit solvent molecules are included in the QM subsystem. We correlate the errors in the 2PA cross sections with the errors in the key electronic properties-identified by the analysis of 2PA natural transition orbitals and 2PA transition densities-such as excitation energies, transition moments, and dipole-moment differences between the initial and final states. Finally, using aqueous PYPb, we investigate the convergence of 2PA cross sections to bulk values.

How to stay out of trouble in RIXS calculations within equation-of-motion coupled-cluster damped response theory? Safe hitchhiking in the excitation manifold by means of core–valence separation

“…In contrast to multi-reference approaches, EOM-CC does not involve systemspecific parameterization (e.g., active-space selection), thus satisfying Pople's requirements of theoretical model chemistry 40 that can be used for systematic studies and comparisons between different systems. The EOM-CC framework yields reliable lowerorder properties such as solvatochromic shifts 41 , transition dipole moments 35 , spin-orbit [42][43][44][45] and non-adiabatic couplings [46][47][48] , as well as higher-order properties 49 such as two-photon absorption cross sections [50][51][52][53][54][55] , static and dynamical polarizabilities [56][57][58][59] . Whereas the bulk of prior developments and applications of the EOM-CC methods as well as of the closely related coupled-cluster response theory [60][61][62] were in the VUV regime, these methods are now being extended to the X-ray regime and their performance is being explored for computing, for example, XAS [15][16][17]19,[63][64][65] , XES 24,66 , and RIXS 24,66 spectra.…”

Implementation of analytic gradients for CCSD and EOM-CCSD using Cholesky decomposition of the electron-repulsion integrals and their derivatives: Theory and benchmarks

“…98 However, no analytic gradients using CD have been so far reported for CC and EOM-CC methods, despite the fact that the CD scheme significantly extends their range of applicability 77,82,83 and in this way renders calculations for medium-size molecules with a few dozens of heavy atoms and 400-800 basis functions feasible on a modest hardware (e.g., midrange single computing nodes). 77,100,101 The present paper addresses this issue and describes our implementation of analytic CCSD and EOM-CCSD gradients when using CD for the treatment of the two-electron integrals. Our work builds upon Ref.…”

Progress in the Theory of X-ray Spectroscopy: From Quantum Chemistry to Machine Learning and Ultrafast Dynamics

“…Building on the approach of Krylov et al that has proven performance for the calculation of valence excited states in EOM-CC, , the fc-CVS-EOM-CCSD scheme is now equipped to address practical problems in XS at the L - and M -edges (Figure ). The proposal of strategies for including environment effects further strengthens these schemes, − closing the gap between theory and reality.…”