Kalyan Kumar Das

Geometries and energies of GeHn and GeH+n (n=1–4)

Complete active space MCSCF (multiconfiguration self‐consistent field) (CASSCF) followed by second‐order configuration interaction (SOCI) and multireference singles and doubles CI (MRSDCI) are carried out on the ground states of GeHn and GeH+n (n=1–4). The equilibrium geometries of these species, adiabatic ionization potentials, and stepwise bond energies [De(Hn−1Ge–H) and De(Hn−1Ge+–H)] are calculated. The ground sate of GeH+4 is a Jahn–Teller distorted 2A1(C2v) state with a GeH+2⋅H2 complex structure. The adiabatic ionization potentials (IPS) of GeHn exhibit even–odd alternation. GeH4 is the most stable among the neutral GeHn species while GeH+3 is the most stable of the GeH+n.

RELATIVISTIC CONFIGURATION INTERACTION STUDY OF THE LOW-LYING ELECTRONIC STATES OF BI2

Relativistic effective core potentials have been employed in the framework of a spin–orbit configuration interaction treatment to compute potential curves, spectroscopic constants, and transition probabilities between pairs of vibrational states of the Bi2 molecule. The calculations find a steady increase in bond length for the lowest four states as a result of successive π→π* excitations en route from the X0+g ground state to the doubly excited 5Σ+g0+g, in good agreement with measured data. The corresponding 1g state with a Te value near 12 000 cm−1 has not yet been located experimentally. The next most stable λ–s state is found to be 3Δu, with Ω components increasing in energy in the order, 2u<3u<1u, of which only the latter has electric dipole‐allowed transitions to X0+g. It is argued that the 1u species should be identified with the observed b state instead of the 3u component, especially since its calculated energy splitting relative to a2u is in much better agreement with the observed b−a separation...

Spin-orbit configuration interaction study of the electronic spectrum of antimony iodide

Abstract Relativistic effective core potentials (RECPs) have been employed in the framework of a spin-orbit configuration interaction (CI) approach to compute potential energy curves for the lowest-lying electronic states of the SbI molecule, as well as the Einstein coefficients of spontaneous emission for transitions between them. In contrast to systems such as arsenic and antimony fluoride, it is found that the lowest 3 II state of SbI is repulsive, and a qualitative explanation for this distinction in terms of the electronegativity difference of the constituent atoms is put forward. The computer T e value of the a 1 Δ 2 state is in good agreement with a result inferred on the basis of experimental b 0 + and 1 Δ 2 T e values in other group VA-VIIA and VIA-VIA systems. Two different semicore and full-core RECPs have been employed at various levels of sophistication in the spin-orbit CI treatment to obtain the present results and the corresponding findings are in generally good agreement with one another. The intensity of the b-X 1 transition is computed to be much larger than that of b-X 2 , in agreement with the observations of Winter et al. The fact that the opposite relationship has been found for SbF and many other diatomics in which the halogen is the lighter of the two atoms, as first pointed out by Colin et al., is discussed and also found to be closely tied up with electronegativity considerations.

Ab initio CI study of the electronic spectrum of bismuth iodide employing relativistic effective core potentials

Abstract A relativistic CI treatment including spin-orbit coupling has been carried out for the low-lying electronic states of bismuth iodide, employing effective core potentials for both atoms. The X 3 Σ − ground state is computed to have a zero-field splitting of 5096 cm −1 , 1086 cm −1 less than the most recent measured values. The a 1 Δ state is predicted to have a T e value of 12336 cm −1 , and it is suggested on the basis of correlation effects that the true value should lie about 1000 cm −1 lower. This conclusion is also based in part on the finding that the computed BO + T e value of 24148 cm −1 overestimates the measured result by 759 cm −1 . The latter state is shown to arise from an avoided crossing between the 1 Σ + and 5 Π Λ - S states, which produces only a relatively shollow well and a slight barrier to dissociation. Because the 3 Π state is repulsive, no other low-lying Ω = 0 + state is found in the spectrum, similarly as in SbI but in contrast to BiF. Due to the much greater spin-orbit effects in BiI, the composition of the lowest two excited 0 + states in terms of 1 Σ + and 3 Π Λ - S states is notably different than in SbI and this fact is important in understanding why the T e value of the lowest bound 0 + states of these two systems are so different. Transition probabilities have also been computed for various pairs of vibrational states. The radiative lifetime of the X 2 1 fine structure component is calculated to be 20.7 ms, which agrees well with a recent measured value of 20 ± 4 ms by Fink and Shestakov. In agreement with Colin et al.s emperical rule, it is found that the b−X 2 transition is stronger than b−X 1 , and this result also confirms an earlier theoretical analysis of this general phenomenon given by the authors.

Ab initio spin-orbit CI calculations of the potential curves and radiative lifetimes of low-lying states of lead monofluoride

The electronic structure of the lead monofluoride molecule is studied by means of ab initio configuration interaction (CI) calculations including the spin-orbit interaction. Potential-energy curves are generated for a large number of electronic states, of which only the X1 2Π1/2 ground and X2 2Π3/2 and A 2Σ+ excited states have been observed experimentally. Two different methods are compared for the inclusion of spin-orbit effects in the theoretical treatment, a contracted CI which employs a basis of large-scale Λ–S eigenfunctions to form a rather small matrix representation of the full relativistic Hamiltonian (two-step approach), and a more computationally laborious technique which involves solution of a secular equation of order 250 000 S2 eigenfunctions of different spin and spatial symmetry to achieve a potentially more evenly balanced description of both relativistic and electron correlation effects (one-step approach). In the present application, it is found that both methods achieve quite good agr...

Ab initio MRD-CI study of the electronic states of the gallium dimer

Low-lying electronic states of are computed using ab initio configuration-interaction (MRD-CI) calculations based on relativistic effective core potentials. Effects of the spin - orbit coupling on the electronic states which are formed from the two ground state atoms studied. We have computed potential energy curves for all 23 states which dissociate into and limits. Spectroscopic constants of the bound states are estimated. The ground state of is originating from the configuration with and . The maximum spin - orbit splitting among the components of is about . The spin - orbit interaction has little effect on the composition of the low-lying states. The dissociation energy of the ground state is calculated to be 1.17 eV. The splitting between and of the atom calculated at the dissociation limits of the molecular state is about compared with the experimental value of .

Ground and excited states of benzil: A theoretical study

Abstract The ground and low-lying excited state potential energy curves of benzil have been studied. The ground state geometry is fully optimized by the AM1 method. The excited states are calculated by using the CNDO/S-CI method. The calculations confirm that benzil in the ground state has a skewed conformation whereas the first excited singlet and triplet states of this molecule are trans-planar. The geometry relaxation in the excited states of benzil agrees well with the experimental findings. The absorption and emission bands of benzil have been compared with the observed bands. The dipole moments of the ground and excited states at some conformations are also reported. The effect of solvents on the electronic states of benzil has been studied using the continuum dielectric model.

Noncovalent interaction assisted fullerene for the transportation of some brain anticancer drugs: A theoretical study

The treatment of brain cancer like glioblastoma multiforme often uses chemotherapeutic drugs like temozolomide, procarbazine, carmustine, and lomustine. Fullerene loaded with these drugs help to cross the blood brain barriers. The adsorptions of the four drug molecules on the surface of the fullerene are studied mostly by using density functional theory (DFT) based method at the M06-2X/6-31G(d) level of calculations. In all four cases, the estimated interactions are noncovalent type and the average adsorption energy lies in between -5 and -11kcal/mol in the gas phase. In the aqueous and protein environment such interactions are weakened further. The binding affinity is further assessed by performing MP2 based calculations to provide interaction energies with a reasonable accuracy. Stabilities and reactivities of the drug adsorbed fullerene complexes are determined from chemical reactivity descriptors. The attached drug molecules increase the polarity of the pristine C60 thus facilitating the drug delivery within the biological systems. The semiconducting behavior of C60 is retained in the C60-drug composite systems. The computed DOS, IR, UV spectra, and molecular orbitals in the vicinity of Fermi level are analyzed to reveal the nature of the noncovalent interactions between C60 and drug molecules. The Wiberg bond order values are used to estimate the strength of the adsorption of the drug molecule on C60. In all four C60-drug interactions, the chemical characteristics of the drug molecule are least perturbed by the C60 moiety thereby suggesting it to be a good carrier for the delivery of these brain anticancer drug molecules to the target cells.

AB INITIO CONFIGURATION INTERACTION CALCULATIONS OF THE POTENTIAL CURVES AND LIFETIMES OF THE LOW-LYING ELECTRONIC STATES OF THE LEAD DIMER

The low‐energy electronic spectrum of the lead dimer is described by means of a multireference configuration interaction treatment based on a semicore relativistic effective core potential (RECP) including spin–orbit coupling. The X0+g ground state is found to be a heavy mixture of the ...σ2π2 3Σ−g, the ...σπ2π* 5Πg and ...π4 1Σ+g Λ–S states, underscoring the importance of the spin–orbit interaction in determining the electronic structure of this heavy system. The first excited state has 1g symmetry and is predominantly 3Σ−g but also with a heavy admixture of 5Πg character. The lowest‐lying excited state as yet observed (A) seems to be the 2u(I) state, however, with a 0.09 A smaller computed re value than for X0+g. The B state with an experimental Te value of 12 457 cm−1 appears to be second 0−u state which arises from an avoided crossing between the ...σπ3 3Πu and the ...σ2ππ* 1Σ−u Λ–S states. Another avoided crossing between the lowest two 0+u states is shown to produce the experimental C and F states, ...

Prediction of binding modes and affinities of 4-substituted-2,3,5,6-tetrafluorobenzenesulfonamide inhibitors to the carbonic anhydrase receptor by docking and ONIOM calculations.

Inhibition activities of a series of 4-substituted-2,3,5,6-tetrafluorobenzenesulfonamides against the human carbonic anhydrase II (HCAII) enzyme have been explored by employing molecular docking and hybrid QM/MM methods. The docking protocol has been employed to assess the best pose of each ligand in the active site cavity of the enzyme, and probe the interactions with the amino acid residues. The docking calculations reveal that the inhibitor binds to the catalytic Zn(2+) site through the deprotonated sulfonamide nitrogen atom by making several hydrophobic and hydrogen bond interactions with the side chain residues depending on the substituted moiety. A cross-docking approach has been adopted prior to the hybrid QM/MM calculation to validate the docked poses. A correlation between the experimental dissociation constants and the docked free energies for the enzyme-inhibitor complexes has been established. Two-layered ONIOM calculations based on QM/MM approach have been performed to evaluate the binding efficacy of the inhibitors. The inhibitor potency has been predicted from the computed binding energies after taking into account of the electronic phenomena associated with enzyme-inhibitor interactions. Both the hybrid (B3LYP) and meta-hybrid (M06-2X) functionals are used for the description of the QM region. To improve the correlation between the experimental biological activity and the theoretical results, a three-layered ONIOM calculation has been carried out and verified for some of the selected inhibitors. The charge transfer stabilization energies are calculated via natural bond orbital analysis to recognize the donor-acceptor interaction in the binding pocket of the enzyme. The nature of binding between the inhibitors and HCAII active site is further analyzed from the electron density distribution maps.