Papers by Uttam Sinha mahapatra
Chemical Physics Letters, 1996
Multireference coupled-cluster (MRCC) theory using an eigenvalue-independent partitioning (EIP) t... more Multireference coupled-cluster (MRCC) theory using an eigenvalue-independent partitioning (EIP) technique is formulated for a general incomplete model space, taking a valence universal normal ordered wave operator 12 = {exp(S)} that acts upon the model space P. For the size-extensivity requirement the intermediate normalization is abandoned (i.e. P12P ~ P) and hence the closed part of O, 12c~, is not unity. This leads to an implicit equation for the Her f. The EIP structure of the MRCC equation is capable of producing the main as well as the satellite states together.
Journal of Computational Chemistry, 2015
Using four-component (4c) relativistic spinors, we present a computationally economical relativis... more Using four-component (4c) relativistic spinors, we present a computationally economical relativistic ab initio method for molecular systems employing our recently proposed secondorder state-specific multireference perturbation theory (SSMRPT) incorporating the improved virtual orbital-complete active space configuration interaction (IVO-CASCI) reference wavefunction. The resulting method, 4c-IVO-SSMRPT [calculate one state at a time] is tested in pilot calculations on the homonuclear dimers including Li 2 , Na 2 , K 2 , Rb 2 , F 2 , Cl 2 , and Br 2 through the computations of the ground state potential energy curves (PECs). As SSMRPT curbs intruder effects, 4c-IVO-SSMRPT is numerically stable. To our knowledge, the SSMRPT in the 4c relativistic framework has not been explored in the past. Selective spectroscopic constants that are closely related to the correct shape and accuracy of the energy surfaces have been extracted from the computed PECs. For the halogen molecules, a relativistic destabilization of the bond has been found. Relativistic and electron correlation effects need to be incorporated to get reliable estimates. Our results are in good accordance with reference theoretical and experimental data which manifests the computational accuracy and efficiency of the new 4c-IVO-SSMRPT method. The method opens for an improved description of MR systems containing heavy elements. The inexpensiveness of IVO-CASCI makes 4c-IVO-SSMRPT method promising for studies on large systems of heavy elements. V
Recent Advances in Spectroscopy, 2010
We have applied multireference (MR) perturbation theory to a degenerate case using the state-spec... more We have applied multireference (MR) perturbation theory to a degenerate case using the state-specific or single root MR approach (SS-MRPT). The objective of this paper is to provide an account of the applications of both Brillouin-Wigner and Rayleigh-Schrodinger versions of the SS-MRPT with Moller-Plesset partitioning of the hamiltonian. The method is very effective to provide an accurate and balanced treatment of dynamical and non-dynamical correlations over a wide range of geometrical parameters of the molecule under study, and hence, seeks to address bond breaking process in an accurate manner. Studies on dissociation potential energy curve and spectroscopic constants of the ground state of F2 are carried out to gauge its accuracy. F2 is a particularly challenging system for electronic structure theory due to strong dynamical and non-dynamical correlation effects. The calculations are compared with other available theoretical results and experimental spectroscopic data. The accuracy of the results appears to be encouraging, taking into account the low computational cost which supports the effectiveness of the method.
International Journal of Molecular Sciences, 2002
We have presented in this paper a new cluster Ansatz for the wave operator for open-shell and/or ... more We have presented in this paper a new cluster Ansatz for the wave operator for open-shell and/or quasidegenerate states, which takes care of strong relaxation and correlation effects in a compact and efficient manner. This Ansatz allows contraction among the various cluster operators via spectator orbitals, accompanied by suitable combinatorial factors. Since both the orbital and the correlation relaxations are treated on the same footing, it allows us to develop a very useful direct method for energy differences for open shell states relative to a closed-shell ground state, where the total charge for the two states may differ. We have discussed a new spin-free coupled cluster (CC) based direct method and illustrated its performance by evaluating electron affinity of a neutral doublet radical. We have also indicated how the scope of the theory can be extended to compute the state energies of simple open shell configurations as well. In that case, the CC equations terminate after the quartic power of cluster operators-exactly as in the closed-shell situation, which is not the case for the current methods.
In this paper we apply our recently developed state-specific multi-reference coupled clu ster bas... more In this paper we apply our recently developed state-specific multi-reference coupled clu ster based linear response method to compute excited state energies for systems whose ground state has a pronounced multi-referen ce character. The parent state-specific theory is bui lt on complete active space reference functions, and is designed to treat quasi-degeneracy of varying degrees while bypassing the intruder problem. The response theory based on such a fun ction offers thus a very conveni ent tool for ge nerating potenti al energy surfaces (PES) for excited states where a traditional response form ul ati on based on a single reference th eory will fail clue to the qu as idegeneracy in some regions of the PES and the effective hamiltonian based response methods woul d be beset by intruders. A succinct summary of the response formalism is presented to both underline the basic issues and the mode of approac h. Illustrati ve numerical results are presented for the PES of th e excited states as a fun cti on of the angle opening for the parad igm system H 4 in trapezoid al geo metry (H4), whose ground state req uires a two-confi gurati on descripti on at the rectangular geo met ry, and has intruders at an intermediate trapezoidal geo metry. A compari so n with th e full Cl results in the same bas is indicates th e effi cacy of th e met hod.
Molecular Physics, 2015
ABSTRACT We present a study on the performance of our iterative triples correction for the couple... more ABSTRACT We present a study on the performance of our iterative triples correction for the coupled cluster singles and doubles excitations (CCSDT-1a+d) method for computation of potential energy surface (PES), spectroscopic constants, and vibrational spectrum for the ground state (X1Σ+) BeMg, where the ostensible inadequacy of the CCSD and CCSD(T) methods is quite expected. We compare our results with those obtained using state-of-the-art multireference configuration interaction (MRCI) investigations reported earlier by Kerkines and Nicolaides. Our estimated dissociation energy (417.37 cm−1), equilibrium distance (3.285 Å), and vibrational frequency (82.32 cm−1) are in good agreement with recent results of advanced MRCI calculations for X1Σ+ BeMg PES, which exhibits a shallow well of 469.4 cm−1 with a minimum at 3.241 Å and a harmonic vibrational frequency of 85.7 cm−1. Very weakly bound nature of X1Σ+ BeMg is clearly reflected from these values. In accord with MRCI studies, a comparison of BeMg with iso-valence weakly bound ground-state species, Be2 and Mg2, suggests that its characteristics do not exhibit any resemblance to Be2 rather, it shows a close kinship to Mg2. The agreement of our derived vibrational levels with those obtained via the high-level MRCI calculations is very encouraging reflecting the potential of the suitably modified single-reference coupled cluster (SRCC) method, CCSDT-1a+d as a tool for the study of multireference van der Waals systems.
Computational Chemistry: Reviews of Current Trends, 2005
... Theories Based on State Specific Multi-Reference Coupled Electron-Pair Approximation Like Met... more ... Theories Based on State Specific Multi-Reference Coupled Electron-Pair Approximation Like Methods Sudip Chattopadhyay, Dola Pahari, Uttam Mahapatra" and ... Pal et al.[35] has developed MR-based LRT based on effective hamiltonian approach in the frame work of the ...
Journal of Molecular Structure: THEOCHEM, 2006
The single reference coupled cluster based linear response theory (CCLRT) and valence-universal m... more The single reference coupled cluster based linear response theory (CCLRT) and valence-universal multi-reference coupled cluster (VU-MRCC) theory for direct difference energy calculations have been presented. The similarities and differences in the structure of these two formalisms have also been addressed. The core-extensive CCLRT is applied to compute the ionization potentials (IPs) of HCl and excitation energies of CuH using Hartree-Fock (HF) orbitals. We further report the relativistic applications of the VU-MRCC theory through the computation of the ground and excited/ionized state energies and related properties of Ag and Hg atoms using Dirac-Hartree-Fock (DHF) orbitals. The IPs, excitation energies, and the spectroscopic constants reported here are in favorable agreement with experiment and with other correlated calculations.
International Journal of Molecular Sciences, 2002
We present in this paper two new versions of Rayleigh-Schrödinger (RS) and the Brillouin-Wigner (... more We present in this paper two new versions of Rayleigh-Schrödinger (RS) and the Brillouin-Wigner (BW) state-specific multi-reference perturbative theories (SS-MRPT) which stem from our state-specific multi-reference coupled-cluster formalism (SS-MRCC), developed with a complete active space (CAS). They are manifestly sizeextensive and are designed to avoid intruders. The combining coefficients c µ for the model functions φ µ are completely relaxed and are obtained by diagonalizing an effective operator in the model space, one root of which is the target eigenvalue of interest. By invoking suitable partitioning of the hamiltonian, very convenient perturbative versions of the formalism in both the RS and the BW forms are developed for the second order energy. The unperturbed hamiltonians for these theories can be chosen to be of both Mφller-Plesset (MP) and Epstein-Nesbet (EN) type. However, we choose the corresponding Fock operator f µ for each model function φ µ , whose diagonal elements are used to define the unperturbed hamiltonian in the MP partition. In the EN partition, we additionally include all the diagonal direct and exchange ladders. Our SS-MRPT thus utilizes a multi-partitioning strategy. Illustrative numerical applications are presented for potential energy surfaces (PES) of the ground (1 Σ +) and the first delta (1 ∆) states of CH + which possess pronounced multi-reference character. Comparison of the results with the corresponding full CI values indicates the efficacy of our formalisms.
Theory and Applications of Computational Chemistry, 2005
This chapter discusses various approaches to state-specific multireference (SSMR) methods which u... more This chapter discusses various approaches to state-specific multireference (SSMR) methods which use both complete and incomplete model spaces (IMSs). These SSMR methods satisfy the desirable criteria: they avoid instabilities due to intruder states in a natural manner; they guarantee size-extensivity of the computed energy; they ensure additivity of the energy for noninteracting subsystems when the model spaces are chosen in a separable manner; and they are manifestly spin-free. For the general case involving open-shell functions in the model space, ones spin-adaptation strategy is to choose them as some suitable configuration state functions (CSF), and use spin-free unitary generators to define the cluster operators. An important innovation in spin-free formulation is the use of the entire portion ф 0μ of the highest closed-shell component of a model function ф μ as the vacuum to define all the excitations on ф μ in normal order.
Physical Review A, 2001
... Geetha Gopakumar, Holger Merlitz, Sonjoy Majumder, Rajat K. Chaudhuri, and BP Das Non-Acceler... more ... Geetha Gopakumar, Holger Merlitz, Sonjoy Majumder, Rajat K. Chaudhuri, and BP Das Non-Accelerator Particle Physics Group, Indian Institute of Astrophysics, Bangalore 560 ... 9 RK Chaudhuri, H. Merlitz, PK Panda, BP Das, Uttam Sinha Mahapatra, and D. Mukherjee, J. Phys. ...
Theoretical Chemistry Accounts, 2012
Assessment of the complete active space-based state-specific multireference Møller-Plesset pertur... more Assessment of the complete active space-based state-specific multireference Møller-Plesset perturbation theory, SS-MRMPPT, has been performed on the ground states of HX (X = F, Cl, and Br) systems through the computation of potential energy surface (PES) and spectroscopic constants (such as equilibrium bond lengths, rotational constants, centrifugal distortion constants, vibrational frequencies, anharmonicity constants, and dissociation energies that are closely related to the shape and accuracy of the energy surfaces) extracted from the computed PES. The SS-MRMPPT (involves multiple amplitude sets to parametrize the exact wavefunction) approach isolates one of the several states provided by an effective Hamiltonian in an attempt to avert intruder states in sizeextensive manner and hence it forms the basis of a robust approach to the electron correlation problem in cases where a multireference formalism is required. The absence of intruder problem makes SS-MRMPPT an interesting choice for the calculation of the dissociation energy surface(s). The performance of the method has been judged by comparing the results with calculations provided by current generation ab initio methods (multireference or singlereference methods) and we found, in general, a very good accordance between them which clearly demonstrates the usefulness of the SS-MRMPPT method.
The Journal of Physical Chemistry A, 1999
... Uttam Sinha Mahapatra, Barnali Datta, and Debashis Mukherjee*. Department of Physical Chemist... more ... Uttam Sinha Mahapatra, Barnali Datta, and Debashis Mukherjee*. Department of Physical Chemistry, Indian Association for the Cultivation of Science, Calcutta 700-032, India. J. Phys. Chem. A , 1999, 103 (12), pp 18221830. ...
The Journal of Physical Chemistry A, 2013
Theoretical investigations of the superheavy elements (SHEs) are extremely challenging and are of... more Theoretical investigations of the superheavy elements (SHEs) are extremely challenging and are often the sole source of useful chemical information. Relativistic Fockspace multireference coupled cluster (RFS-MRCC) computations have been carried out for evaluating the ionization potential (IP), excitation energies (EE), nuclear magnetic hyperfine constant (A), lifetime (τ), and Landég factor of singly ionized eka-lead (Fl II). To judge the accuracy of Fl II results, similar calculations are performed for Pb II, which shows a nice and consistent agreement with known experimental values. Thus, we believe that our predictions for Fl are reliable and useful for the simulation of experimental behavior. To the best of our knowledge, no prior theoretical and/or experimental information is available for A, τ, and gfactor of this SHE. The higher IPs and EEs of Fl II, with respect to Pb II, indicate the former to be more inert and less metallic than Pb. This is contingent on the effects of the relativistic stabilization of the 7s and 7p 1/2 orbitals. The present analysis demonstrates the influence of higher-body cluster operators on atomic properties. The close agreement with the experiment (having an estimated error within 1−2%) indicates that the FS-MRCC method is a reliable predictive tool in cases where the experimental results are not readily available, such as the SHEs. The remaining source of error possibly stems out from the omission of the full-blown triple virtual excitations and the absence of Breit interaction.
The Journal of Physical Chemistry A, 2009
The relative performance of four variants of the Møller-Plesset (MP) partitioning (using differen... more The relative performance of four variants of the Møller-Plesset (MP) partitioning (using different diagonal one-electron unperturbed Hamiltonian, H0) based state-specific multireference perturbation theory (SS-MRPT) [termed as SS-MRPT(MP)] has been investigated and demonstrated by calculations of the dissociation potential energy curves (PECs) of the first three electronic states [ground state X3Sigmag- as well as low-lying singlet excited states, a1Deltag and b1Sigmag+] of the oxygen molecule using different basis sets. The spectroscopic constants extracted from the computed PECs obtained by the SS-MRPT(MP) method are calibrated with respect to the corresponding value of the full configuration interaction (FCI) and experimental data for the corresponding states. We have also computed vertical excitation (or transition) energies and compared those with the corresponding FCI values along with the results of other available sophisticated methods. Encouraging agreement between SS-MRPT(MP) theory and some benchmark calculations has been observed. We have thus assessed the applicability and accuracy of the SS-MRPT(MP) method with different diagonal one-electron partitioning schemes. The ability of the SS-MRPT(MP) method with different partitioning schemes to predict full PECs and spectroscopic constants of the ground state and excited states with almost equivalent accuracy is promising.
The Journal of Physical Chemistry A, 2013
The coupled cluster based linear response theory (CCLRT) with fourcomponent relativistic spinors ... more The coupled cluster based linear response theory (CCLRT) with fourcomponent relativistic spinors is employed to compute the electric field gradients (EFG) of 35 Cl, 79 Br, and 127 I nuclei. The EFGs resulting from these calculations are combined with experimental nuclear quadrupole coupling constants (NQCC) to determine the nuclear quadrupole moments (NQM), Q of the halide nuclei. Our estimated NQMs [ 35 Cl = −81.12 mb, 79 Br = 307.98 mb, and 127 I = −688.22 mb] agree well with the new atomic values [ 35 Cl = −81.1(1.2), 79 Br = 302(5), and 127 I = −680(10) mb] obtained via Fock space multireference coupled cluster method with the Dirac−Coulomb−Breit Hamiltonian. Although our estimated Q(79 Br) value deviates from the accepted reference value of 313(3) mb, it agrees well with the recently recommended value, Q(79 Br) = 308.7(20) mb. Good agreement with current reference data indicates the accuracy of the proposed value for these halogen nuclei and lends credence to the results obtained via CCLRT approach. The electron affinities yielded by this method with no extra cost are also in good agreement with experimental values, which bolster our belief that the NQMs values for halogen nuclei derived here are reliable.
The Astrophysical Journal, 2002
One of the most successful ab initio, highly correlated all-order many-body methods, the relativi... more One of the most successful ab initio, highly correlated all-order many-body methods, the relativistic coupled cluster theory, is employed to calculate excitation energies of the doublet states of Mg + and allowed transitions among them that are of interest in astrophysical problems. We have also calculated oscillator strength for the 3s-4p doublet transitions, which is improved over the existing results. These transition lines have been sought after in astronomical observations because they represent the best column density identifier in the interstellar medium. Our calculated oscillator strength (9:3 Â 10 À4) and branching ratio (1.80) of these doublet lines matches well with the recent empirical and semiempirical calculations. Subject headings: atomic data-methods: analytical
Physics of Plasmas, 2012
We report the plasma screening effect on the first ionization potential (IP) and ½He2s 2 ð 1 S 0 ... more We report the plasma screening effect on the first ionization potential (IP) and ½He2s 2 ð 1 S 0 Þ ! ½He2s2p=2s3p allowed (1 P 1) and inter-combination transitions (3 P 1) in some selected Be-like ions. In addition, we investigate the spectral properties of ½Ne3s ð 2 S 1=2 Þ ! ½Nenp ð 2 P 1=2 and 2 P 3=2 for n ¼ 3, 4) transitions in Ca X and Fe XVI ions (Na I isoelectronic sequence) and ½He3sð 2 S 1=2 Þ ! ½Henp ð 2 P 1=2 and 2 P 3=2 for n ¼ 2, 3) transitions in Li, B II, and N IV (Li I isoelectronic sequence) under plasma environment. The state-of-the-art relativistic coupled cluster calculations using the Debye model of plasma for electron-nucleus interaction show that (a) the ionization potential decreases sharply with increasing plasma strength and (b) the gap between the ½He2s 2 ð 1 S 0 Þ ! ½He2s2pð 1;3 P 1 Þ energy levels increases with increasing plasma potential and nuclear charge. It is found that the ½He2s 2 ð 1 S 0 Þ ! 2s3p ð 1;3 P 1 Þ transition energy decreases uniformly with increasing plasma potential and nuclear charge. In other words, the spectral lines associated with 2s-2p (i.e., Dn ¼ 0, where n corresponds to principle quantum number) transitions in Be I isoelectronic sequence exhibit a blue-shift (except for Be I, B II, and the lowest inter-combination line in C III, which exhibit a red-shift), whereas those associated with 2s-3p (i.e., Dn 6 ¼ 0) transitions are redshifted. Similar trend is observed in Li I and Na I isoelectronic sequences, where spectral lines associated with Dn ¼ 0 ðDn 6 ¼ 0Þ are blue-shifted (red-shifted). The effect of Coulomb screening on the spectral lines of ions subjected to plasma is also addressed. V
Physical Review A, 2001
Ionization potentials ͑IP's͒ are evaluated for various excited states of Tl using the relativisti... more Ionization potentials ͑IP's͒ are evaluated for various excited states of Tl using the relativistic coupled cluster ͑CCCD͒ theory in the even-parity pair channel approximation ͑CCSD-EPC͒. An average accuracy below half a percent is reached. The effect of deep core electrons on the core-valence correlations is investigated. It is found that electrons in the third subshell (nϭ3) modify the IP's of the 6p orbitals by 100 cm Ϫ1. By comparison with calculations made in the linearized CCSD ͑LCCSD͒ approximation it is demonstrated that nonlinear contributions are mandatory to reach an accuracy below half a percent for the 6p 1/2 orbital.
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Papers by Uttam Sinha mahapatra