Reappraisal of Nuclear Quadrupole Moments of Atomic Halogens via Relativistic Coupled Cluster Linear Response Theory for the Ionization Process Rajat K. Chaudhuri* Indian Institute of Astrophysics, Bangalore 560034, India Sudip Chattopadhyay* Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711103, India Uttam Sinha Mahapatra Department of Physics, Maulana Azad College, 8 Rafi Ahmed Kidwai Road, Kolkata 700 013, India ABSTRACT: The coupled cluster based linear response theory (CCLRT) with four- component 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. I. INTRODUCTION The relativistic quantum chemical methodology finds one of its most important domain of applications in the evaluation of molecular properties. 1-4 The analytic evaluation of molecular response properties calls for the analytic derivatives of the molecular energy, and in this regard, the generation of analytic energy gradients (and Hessians) are considered to be one of the esteemed ways for reaching molecular properties within the domain of traditional many-body approaches. 5 It has become quite popular and standard to perform quantum chemical calculations of electric response properties with nonrelativistic methods. 5 However, quite contrary to this, the study of the domain of systems involving heavy elements is comparatively less explored, because such systems warrant a truly relativistic description. 1-4 The formal complexity and extensively involved numerical implementation of complete four-component relativistic many-body formulations restrict the applicability of such methods to small systems only 6 and motivates one to look for simple but accurate computational strategies. The nuclear quadrupole moment (NQM) that emerges due to the nonspherical distribution of the nuclear charge plays an important role in atomic, molecular, and solid state spectros- copy 7-18 besides the direct interest in nuclear physics, where its determination can be used to check nuclear models. Moreover, investigations of molecular dynamics require the information of Q in systems where NQM values determine the spin-lattice relaxation time. The information of NQM (assigned here as Q) is also useful in the evaluation of the nuclear magnetic resonance measurements in biological systems. Currently, the nuclear quadrupole moment Q of an atom is determined by exploiting the following relation 9 = − × Q B q [b] [MHz] 234.9647 [a.u.] (1) where q is the electric field gradient (EFG) and B is the nuclear quadrupole coupling constant (NQCC). As the nuclear quadrupole coupling constant can be determined experimen- tally with high precision, 19-23 the accuracy of the nuclear quadrupole moment Q derived from eq 1 solely relies on the accuracy of the computed EFG, q. Therefore, the error associated with Q can be attributed to the uncertainty in the computation of EFG value. The EFG tensor at the location of Received: August 29, 2013 Revised: October 25, 2013 Published: October 30, 2013 Article pubs.acs.org/JPCA © 2013 American Chemical Society 12616 dx.doi.org/10.1021/jp408645g | J. Phys. Chem. A 2013, 117, 12616-12627