Department of Chemistry

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Author: search.filters.author.Manohar, Prashant Uday

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    Equation-of-motion coupled-cluster singles, doubles and(full) triples for doubly ionized and two-electron-attached states: a computational implementation
    (2025-01) Manohar, Prashant Uday
    We present our computational implementation of the equation-of-motion (EOM) coupled-cluster (CC) singles, doubles, and triples (SDT) method for computing doubly ionized (DIP) and two-electron attached (DEA) states within Q-CHEM. These variants have been implemented within both the (conventional) double precision (DP) and the single precision (SP) algorithms and will be available in the upcoming major release of {\sl Q-CHEM}. We present here the programmable expressions and some pilot application of for DIP and DEA EOM-CCSDT.
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    Spin–flip equation-of-motion coupled cluster method with singles, doubles and (full) triples: computational implementation and some pilot applications
    (RSC, 2024-07) Manohar, Prashant Uday
    We present our computational implementation of the spin–flip (SF) equation-of-motion (EOM) coupled-cluster (CC) method with singles, doubles, and (full) triples (SDT) within Q-CHEM. The inclusion of triples not only enhances the quantitative accuracy of the SF-EOM-CCSD method but also provides correct qualitative trends in the energy gaps between strongly degenerate states. To assess the accuracy, we compare our SF-EOM-CCSDT results with full configuration interaction (FCI) and complete-active-space self-consistent field second-order (CASSCF-SO) CI benchmarks to study the adiabatic energy gaps in CH2 and NH2+ diradicals, vertical excitation energies in CH radicals and the bond dissociation of the HF molecule. We have implemented SF-EOM-CCSDT using both the conventional double precision (DP) and the single precision (SP) algorithms. The use of SP does not introduce any significant errors in energies and energy gaps, and, due to low cost (relative to DP), turns out to be a promising approach to widen the applicability of EOM-CCSDT to bigger molecules.
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    On Some Aspects of Fock-Space Multi-Reference Coupled-Cluster Singles and Doubles Energies and Optical Properties
    (Springer, 2010-04) Manohar, Prashant Uday
    Multi-reference coupled cluster methods are established as accurate and efficient tools for describing electronic structure of quasi degenerate states. Recently we have developed multi-reference coupled cluster linear response approach based on the constrained variation method. The method is very general and can describe challenging problems due to the multiple-root nature of effective Hamiltonian. Calculation of response properties for the ionized/electron attached or excited state molecules is a challenging task. With this formulation it is possible to accurately predict the higher order molecular properties of the open shell molecules. In this article we review the response approaches for quasi degenerate cases with emphasis on Fock space multi-reference coupled cluster method.
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    Extended coupled-cluster approach for magnetizabilities of small molecules
    (Elsiever, 2004-04) Manohar, Prashant Uday
    Extended coupled-cluster method has emerged as a compact method for study of energies and electric properties of molecules and extended systems. In this Letter, we present a linear response to the above method for study of magnetizabilities of small test molecules. Specifically, we have presented results of diamagnetic and paramagnetic magnetizabilities of HF and CO in appropriate basis sets and studied the gauge dependence and basis set dependence of the properties evaluated through the above method. The results have been compared with SCF and NCC values, as well as experimental values.
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    Electronic Spectra and Ionization Potentials of Halogen Oxides Using the Fock Space Coupled-Cluster Method
    (2005) Manohar, Prashant Uday
    The ionization potentials and excitation energies of halogen monoxides and dihalogen oxides are studied using fully size-extensive Fock space version of multi-reference coupled-cluster theory. The low-lying excited states of ClO-, FO-, Cl2O and F2O are obtained along with the ionization spectra of Cl2O and F2O. The adiabatic electron affinity of ClO, FO and the vertical detachment energy of ClO- and FO- are also presented.
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    Constrained variational response to Fock-space multi-reference coupled-cluster theory: Some pilot applications
    (Elsiever, 2006-08) Manohar, Prashant Uday
    Fock space (FS) multi-reference (MR) coupled-cluster (CC) has been established to be state-of-the-art method for energies of open-shell radicals, ionized, electron attached states and excited states. Due to the multi-root nature of the method, the linear response of MRCC, however, is non-trivial. Constrained variation approach (CVA) makes it plausible to calculate response of the MRCC method efficiently. However, the approach is used for only one root of the problem at a time. In the present article, we make first implementation of the approach and present test results of dipole moments of small radicals.
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    Dipole moments and polarizabilities of some small radicals using constrained variational response to Fock-space multi-reference coupled-cluster theory
    (Elsiever, 2007-04) Manohar, Prashant Uday
    In this Letter, we present dipole moments and polarizabilities of some open-shell doublet radicals, obtained analytically using constrained variational response to Fock-space (FS) multi-reference (MR) coupled cluster (CC) theory. These radicals can be treated as electron attached (EA) states of the corresponding closed-shell cations. Additionally, we also report analytical polarizabilities of some doublet radicals, which can be considered as ionized (IP) states of the corresponding anions. We compare our results with finite field FSMRCC response and the available benchmark results.
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    Analytical Dipole Moments and Dipole Polarizabilities of Oxygen Mono-Fluoride and Nitrogen Dioxide: A Constrained Variational Response to Fock-Space Multi-Reference Coupled-Cluster Method
    (2007) Manohar, Prashant Uday
    In this article, we present analytical dipole moments and dipole polarizabilities of oxygen mono°uoride and nitrogen dioxide using the recently developed constrained vari- ational approach in Fock-space multi-reference coupled-cluster singles and doubles (CVA- FSMRCCSD) method. Both these molecules are important in atmospheric chemistry. The near-degeneracy in low-lying states of these molecules demands for MR description of the wavefunction. We also report our study of variation in the properties of oxygen mono°u- oride with respect to basis set. Benchmark results, wherever available, have been duly reported for comparison.
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    Constrained Variational Response to Fock‐Space Multi‐Reference Coupled‐Cluster Theory: Formulation for Excited‐State Electronic Structure Calculations and Some Pilot Applications
    (AIP, 2007-12-03) Manohar, Prashant Uday
    Fock‐space (FS) multi‐reference (MR) coupled‐cluster (CC) method has emerged as compact tool to account for electronic structure of open‐shell systems and molecules in low‐lying excited states. Development of linear response (LR) has been one of the challenging problems in FSMRCC due to multiple‐root nature of effective Hamiltonian. The recently developed constrained variational approach (CVA) has opened up a promising tool for efficient evaluation of analytic response properties. In this article, we present formulation of the method for excited state calculations. We discuss the decoupling of equations as a result of spin‐adaptation and present some preliminary results for analytical dipole moments and polarizabilities of some molecules in low‐lying triplet excited states.
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    A noniterative perturbative triples correction for the spin-flipping and spin-conserving equation-of-motion coupled-cluster methods with single and double substitutions
    (AIP, 2008-11-18) Manohar, Prashant Uday
    A noniterative 𝑁7 triples correction for the equation-of-motion coupled-cluster method with single and double substitutions (CCSD) is presented. The correction is derived by second-order perturbation treatment of the similarity-transformed CCSD Hamiltonian. The spin-conserving variant of the correction is identical to the triples correction of Piecuch and co-workers [Mol. Phys. 104, 2149 (2006)] derived within method-of-moments framework and is not size intensive. The spin-flip variant of the correction is size intensive. The performance of the correction is demonstrated by calculations of electronic excitation energies in methylene, nitrenium ion, cyclobutadiene, ortho-, meta-, and para-benzynes, 1,2,3-tridehydrobenzene, as well as C–C bond breaking in ethane. In all cases except cyclobutadiene, the absolute values of the correction for energy differences were 0.1 eV or less. In cyclobutadiene, the absolute values of the correction were as large as 0.4 eV. In most cases, the correction reduced the errors against the benchmark values by about a factor of 2–3, the absolute errors being less than 0.04 eV.