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[en] Antiferromagnetic and dx2-y2-pairing correlations appear delicately balanced in the two-dimensional Hubbard model. Whether doping can tip the balance to pairing is unclear and models with additional interaction terms have been studied. In one of these, the square of a local hopping kinetic energy HW was found to favor pairing. However, such a term can be separated into a number of simpler processes and one would like to know which of these terms is responsible for enhancing the pairing. Here we analyze these processes for a two-leg Hubbard ladder. (c) 2000 The American Physical Society
[en] A new ansatz for local electron correlation is introduced, which truncates double substitutions subject to a triatomics in molecules (TRIM) criterion. TRIM includes all double substitutions in which one occupied-virtual substitution is atomic while the other substitution can be nonlocal (a cubic number, before cutoffs). With an additional approximation, the TRIM second-order Moeller-Plesset perturbation theory (MP2) model can be noniteratively solved; this is the model that is implemented. Results are shown for absolute energies of alkane and polyene chains, rotational barriers of substituted ethylenes and benzenes, and association energies of the water and neon dimers. Over 99.7% of the untruncated MP2 energy is recovered for the test cases, and the relative energies of small systems are in error by less than 0.1 kcal/mol. By contrast, a diatomics in molecules (DIM) truncation recovers about 95% of the full MP2 energy, and yields errors several times larger for relative energies. (c) 2000 American Institute of Physics
[en] Highlights: • Diffuse electron systems. • Vertical ionization energy. • Basis set effect. • Electron correlation (MP2, CC). Present investigation reveals that diffuse electron compounds M2X predict large (106 a.u.) γav value. The basis set dependence of γav showed that the quality of basis function rather than its size is more important. Sadlej’s Pol basis set which is nearly four times less in size than aug-cc-pVQZ can give reliable estimate of γav. MP2 method in conjunction with large basis sets can give comparable results of γav that could be obtained from CCSD or CCSD(T). Higher order energy correction to MP2 leads to oscillatory behavior but inclusion of triple excitations to CCSD improves the magnitude of γav.
[en] The small-rsub(s) expansion is derived for the positron correlation energy and annihilation rate in a homogeneous electron gas. The correlation energy is found to be μ = -1.563 x rsub(s)sup(-1/2) + 0.051 (ln rsub(s))2 -0.098 ln rsub(s) + 0.976 + higher terms. (author)