Calculation of static molecular properties in the framework of the unitary group based coupled cluster approach

Abstract

The recently developed and implemented state selective, fully spin-adapted coupled cluster (CC) method that employs a single, yet effectively multiconfigurational, spin-free reference and the formalism of the unitary group approach (UGA) to the many-electron correlation problem, has been employed to calculate static electric properties of various open-shell ( 0 s ) systems using the finite field (FF) technique. Starting with the lithium atom, the method was applied at the first-order interacting space single and double excitation level (CCSD(is)) to several first-and second-row hydrides having OS ground state, namely to the CH, NH, OH, SiH, PH, and SH radicals. In the case of NH we also considered three OS excited states. In all cases the dipole moment and polarizability were determined using a high quality basis set and compared with the experiment, whenever available, as well as with various configuration interaction results and other theoretical results that are available from the literature. The agreement of our CCSD(is) values with experiment is very satisfactory except for the 3~-ground state of the NH radical, where the experimentally determined dipole moment is too small. No experimental data are available for the corresponding polarizabilities. It is also shown that the FF technique is not suitable for calculations of higher order static properties, such as the hyperpolarizability P and y tensors. For this reason we formulate the linear response version of our UGA-based CCSD approach and discuss the aspects of its future implementation.Key words: static molecular properties, dipole moments, polarizabilities, free radicals, unitary group based coupled cluster method, linear response theory, finite field technique.RCsurnC : On a utilisk la mtthode des cccoupled cluster,, (((CCn) complktement adaptCe aux spin et sClective des Ctats, dCveloppCe et mise en place rkcemment, qui n'utilise qu'une rCfCrence sans spin, mais qui est malgrC tout effectivement multiconfigurationnelle, ainsi que le formalisme de groups unitaires (NUGAD) au problkme de la corrClation de plusieurs Clectrons pour calculer les propriCtCs statiques de divers systernes en couches ouvertes (c<OS))) en utilisant la technique du champ fini (<<FFn). En cornmencant avec I'atome de lithium, on a appliquC cette mCthode au premier niveau d'interaction de l'espace des configurations singulikrernent et doublement excitCes (CCSD(is)) pour plusieurs hydrures des deux premikres couches possCdant un Ctat fondamental OS, soit les radicaux CH, NH, OH, SiH, PS et SH. Dans le cas du NH, on a aussi considCr.5 trois Ctats excitCs OS. Dans tous les cas, on a determini le moment dipolaire et la polarisabilitC en utilisant un ensemble de base de bonne quantitC et, dans tous les cas oh cela Ctait possible, on les a comparCs avec les donnCes expirimentales ainsi qu'avec les divers rCsultats d'interactions de configuration et d'autres risultats theoriques disponibles dans la 1ittCrature. L'accord entre nos valeurs de CCSD(is) et les valeurs expkrimentales...

Time-Domain Measurement of Spontaneous Vibrational Decay of Magnetically Trapped NH

“…The result, 1.5246 D, is 10% larger than the 1974 experimental value of 1:38 0:07 D [32] but it agrees well with the 1996 ab initio value of 1.536 D Paldus and Li [33], who already express doubt about the experimental value. PRL 100, 083003 (2008) P H Y S I C A L R E V I E W L E T T E R S week ending 29 FEBRUARY 2008 083003-3 While both our measured and calculated values for the transition dipole moment agree, they differ significantly from previously published values.…”

Unitary group based open-shell coupled cluster theory: Application to van der Waals interactions of high-spin systems

“…25 for a historical perspective͒ has been fully developed and extensively tested by Paldus group. 29,[36][37][38][39][40][41][42][43][44][45] Various applications of the resulting approach, referred to as the UGA CC method, showed its viability and usefulness in calculations of potential energy surfaces, excitation energies, ionization potentials, force fields, and electric properties of molecules, see Refs. 5 and 25 for comprehensive reviews.…”

A Critical Assessment of Coupled Cluster Method in Quantum Chemistry

“…For example, for the three-electron Li atom, the polarizability obtained with a UGA CCSD and ANO/spdf basis is 165.1 a.u., which agrees well with the experimental value of 164h3 a.u. [277] and is identical to the FCI value for the same basis. In Table 20, we present the hyperpolarizabilities for the H F molecule obtained by Kondo et al [321] with a POLl basis and by Hattig et al [121] with a considerably larger t-aug-cc-pVTZ basis.…”

“…This is also the case for a perturbative account of triples [273,279], representing 0s generalizations of the CCSD(T) and CCSD[T] procedures, which, however, can be employed only near the equilibrium geometries, as in the SR case. Various applications of UGA CCSD to potential-energy surfaces [44,45,200,206,232,239,2721, vertical excitation energies [232,274,276], ionization potentials [44,275], harmonic force fields [278], and dipole moments and polarizabilities [44,277] show the viability and usefulness of this approach and will be compared with the results obtained by other methods in Section 111.…”

“…The UGA CCSD method was also employed to predict static properties of the excited state [277]. This is illustrated in Table 19 Polarizabilities and hyperpolarizabilities are often crucially dependent on dynamical correlation effects.…”