18 August 2000 Ž . Chemical Physics Letters 326 2000 269–276 www.elsevier.nlrlocatercplett On the molecular electric quadrupole moment and the electric-field-gradient-induced birefringence of CO and CS 2 2 Sonia Coriani a, ) , Asger Halkier b , Antonio Rizzo c , Kenneth Ruud d a ˚ Department of Chemistry, Aarhus UniÕersity, Langelandsgade 140, DK-8000 Arhus C, Denmark b Theoretical Chemistry Group, Debye Institute, Utrecht UniÕersity, P.O.Box 80052, NL-3508 TB Utrecht, The Netherlands c Istituto di Chimica Quantistica ed Energetica Molecolare del C.N.R, Area della Ricerca di Pisa, loc. San Cataldo, I-56010 Ghezzano, San Giuliano Terme, Pisa, Italy d San Diego Supercomputer Center and Department of Chemistry and Biochemistry, 9500 Gilman DriÕe, La Jolla, CA 92093-0505, USA Received 19 May 2000; in final form 16 June 2000 Abstract An ab initio investigation of the quadrupole moment, the polarizability anisotropy and the temperature-independent term Ž . entering the electric-field-gradient-induced birefringence EFGB of CO and CS is presented. For the latter property, the 2 2 results allow us to analyze numerically the differences between two semiclassical theories of the EFGB for nonpolar systems. Accurate electric quadrupole moments are obtained from the combined use of coupled cluster methods and Ž . y40 diffusely augmented correlation-consistent basis sets. For CO , our best ab initio estimate of Q sy14.29 "0.09 =10 2 0 2 Ž . y40 2 Cm is in excellent agreement with the two most recent experimentally deduced results y14.3 "0.6 =10 Cm and Ž . y40 2 y14.27 "0.61 =10 Cm . q 2000 Elsevier Science B.V. All rights reserved. 1. Introduction The molecular electric quadrupole moment is a property widely studied both theoretically and exper- imentally. It is of particular importance for nonpolar molecules, where it represents the first non-vanishing electric moment and thus dominates the interaction between the molecule and an external non-uniform electric field. A common route to experimentally determine the molecular electric quadrupole moment consists of measuring the linear birefringence in- ) Corresponding author. Permanent address: Dipartimento di Scienze Chimiche, Universita degli Studi di Trieste, via Giorgieri ` 1, I-34127 Trieste, Italy. Fax: q 45-8619-6199; e-mail: sonia@kemi.aau.dk duced by an electric field gradient on an initially Ž linearly polarized light beam passing through a low . density fluid sample. The method was proposed wx some forty years ago by Buckingham 1 , and it has Ž been successfully applied to a variety of systems see wx . e.g. Ref. 2 for a review on the subject . Within the semiclassical formulation of the elec- Ž . tric-field-gradient-induced birefringence EFGB by w x Buckingham and coworkers 1,3,4 , the induced ani- sotropy of the refractive index for axially symmetric Ž nondipolar gaseous molecules with principal axis . along the z axis at low constant pressure can be written as NF X 2 Dn s b q QD a , 1 Ž. ½ 5 2 e 15kT 0 0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 00 00793-4