It is demonstrated that the “corrected Hartree–Fock” (CHF) density matrix functional proposed by Csanyi and Arias is identical with the Hartree–Fock–Bogoliubov (HFB) functional of the generalized density matrix up to the sign of the pairing energy term. Using this analogy, variational CHF calculations can be performed much more efficiently by solving the HFB equations for the generalized density matrix than by optimizing separately the natural orbitals and their occupations numbers. A family of CHF-type functionals with a scaled pairing energy is introduced and compared to the closely related antisymmetrized geminal power method.

Optimization of density matrix functionals by the Hartree–Fock–Bogoliubov method

Deuterium quadrupole coupling constants have been determined by distant ENDOR spectroscopy for a series of carboxylic acids in which deuterium is substituted at the H-bond position. The measured coupling constants cover a range of 40 to 175 kHz corresponding to a variation of the bridge (O-H … O) length of 2·45 to 2·70 A. Features of the electronic and geometrical structure which principally determine e 2 qQ and η of deuterium in these H-bond situations have been assessed. To aid this assessment a simple model is proposed on which theoretical calculations of the INDO MO type are based. Good agreement between experiment and theory is obtained. It is concluded that for calculation of the electric-field gradient tensor two and three-centre integrals in the vicinity of the H-bond cannot be neglected, and further, in the cases of short and symmetrical H-bonds, e 2 qQ can change sign. Our studies also indicate that in non-ionic systems the asymmetry parameter η is primarily determined by the molecule itself, ra...

Deuterium quadrupole coupling constants in hydrogen bonded dicarboxylic acids

In this paper, we analyse the effects on the nuclear quadrupole coupling behaviour of 2 H and 17 O nuclei of a shift in H-bond character from asymmetric to symmetric. Using ab initio methods, the coupling amplitudes (nuclear quadrupole coupling constant e 2 q Q/h ) coupling anisotropies (asymmetry parameter η), and orientations of the electric field gradient (EFG) principal axis (PA) system of the H-bonded deuterium and oxygen nuclei in the formic acid dimer and related monomers, and the deuterium diformate anion are calculated. In addition, the relative contributions to the 2 H and 17 O EFGs of nuclear and electronic terms, and also the convergence of the EFG as a function of contributions from increasingly distant nuclei in the molecule are investigated. The trends in the calculated 17 O EFGs on going from an asymmetric to a symmetric H-bonded environment are correlated with experimental nqr data in order to establish the hitherto unknown e 2 qQ/h sign, EFG assignments and PA orientation of symmetrically H-bonded oxygen. The possibility that the e 2 qQ/h value of deuterium is negative for symmetric H bonds is discussed, and difficulties in the computation of 2 H EFGs for symmetric - as distinct from asymmetric - H-bonded systems are pointed out. In strong disagreement with assumptions in the literature, it is found that nearest neighbour terms do not dominate the 2 H EFG in symmetrically H-bonded systems.

Comparison of the effects of symmetric versus asymmetric H bonding on 2H and 17O nuclear quadrupole coupling constants: Application to formic acid and the hydrogen diformate anion

Deuteron magnetic resonance study of kd(CF3COO)2

The energy functional of a novel electronic structure theory, OP-NSOFT, has as variables the natural spin orbitals (NSO) of the trial function and their joint occupation probabilities in the search for the ground state energy. When occupancy is restricted to the spin-paired NSOs of DOCI, the resulting theory, OP-NSOFT-0, scales as M3, with M the size of the one-electron basis set. Accurate results were obtained for small molecules, particularly near dissociation where single reference theories like DFT are inaccurate. The homogeneous electron liquid (HEL) could serve as a test bed of OP-NSOFT for condensed systems, but OP-NSOFT-0 reduces to the Hartree–Fock approximation for the HEL. Cooper pairing is introduced instead, both singlet pairing, OP-NOFT-Cs, and fully polarized triplet pairing, OP-NSOFT-Ct. The former yields 1/3 of the diffusion-Monte-Carlo correlation energy, the latter 1/2 to 1/3 with decreasing electron density for rs values between 1 and 10. Both yield the discontinuity in the single-particle occupation number required by the Luttinger theorem. Two-state joint occupation probabilities illustrate the importance of electron–electron small-angle scattering in establishing electron correlation in the unpolarized HEL.

Occupation probabilities as variables in electronic structure theory: cooper pairing, OP-NSOFT-Cs,t, and the homogeneous electron liquid

Calculations on molecular systems with the Hartree-Fock-Bogoliubov self-consistent-field method

Deuteron quadrupole coupling constants for the free formic and trifluoroacetic acid molecules, for their acid anions and for the maleate anion were calculated using the INDO method. The results are compared with experimental values for potassium deuterium maleate and bistrifluoroacetate.

Deuteron quadrupole coupling constants in crystals with symmetrical hydrogen bonds

The influence of the presence of fractionally charged particles on the ionization potentials of molecules is studied by an ab-initio method.

J. Koller

Papers

Calculations on molecular systems with the Hartree-Fock-Bogoliubov self-consistent-field method

Deuteron quadrupole coupling constants in crystals with symmetrical hydrogen bonds

Influence of a quark on molecular ionization potentials