Computation and analysis of the full configuration interaction wave function of some simple systems

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The Total Position Spread (TPS) tensor, defined as the second moment cumulant of the position operator, is a key quantity to describe the mobility of electrons in a molecule or an extended system. In the present investigation, the partition of the TPS tensor according to spin variables is derived and discussed. It is shown that, while the spin-summed TPS gives information on charge mobility, the spin-partitioned TPS tensor becomes a powerful tool that provides information about spin fluctuations. The case of the hydrogen molecule is treated, both analytically, by using a 1s Slater-type orbital, and numerically, at Full Configuration Interaction (FCI) level with a V6Z basis set. It is found that, for very large inter-nuclear distances, the partitioned tensor growths quadratically with the distance in some of the low-lying electronic states. This fact is related to the presence of entanglement in the wave function. Non-dimerized open chains described by a model Hubbard Hamiltonian and linear hydrogen chains H n (n ≥ 2), composed of equally spaced atoms, are also studied at FCI level. The hydrogen systems show the presence of marked maxima for the spin-summed TPS (corresponding to a high charge mobility) when the inter-nuclear distance is about 2 bohrs. This fact can be associated to the presence of a Mott transition occurring in this region. The spin-partitioned TPS tensor, on the other hand, has a quadratical growth at long distances, a fact that corresponds to the high spin mobility in a magnetic system. C 2015 AIP Publishing LLC. [http://dx

Section: A Computational Detailsmentioning

confidence: 99%

The total position-spread tensor: Spin partition

Khatib

Brea

Fertitta

et al. 2015

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“…The FCI calculations have been performed with the VEGA code developed by some of us. 46,47 The VEGA program was adapted to use the MOLCOST code 48 of the Toulouse group that provides an interface with MOLCAS. Three cut sections of the potential energy hypersurface have been calculated. The first one corresponds to the symmetrical atomization dissociation from the equilateral triangle structure of the minimum to the complete dissociation of the cluster.…”

Section: Computational Detailsmentioning

confidence: 99%

Full configuration interaction calculation of Be3

Junquera‐Hernández

Sánchez‐Marín

Bendazzoli

et al. 2004

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The full configuration interaction (FCI) study of the ground state of the neutral beryllium trimer has been performed using an atomic natural orbitals [3s2p1d] basis set. Both triangular and linear structures have been considered for the Be(3) cluster. The optimal geometry for the equilateral triangle has been calculated. The potential energy cut sections along the normal a(1)(') mode and one of the components of the e(') mode have then been studied. The FCI symmetric atomization potential of the linear cluster is also reported. It shows a secondary van der Waals minimum at a long bond distance. All singular points in the potential energy curves are characterized. Other properties, like dissociation energies D(e) and vibrational frequencies, have been estimated from a fourth-order fitting of a large range of points around the minima. The calculated FCI wave number values for the nu(1) and nu(2) normal modes are (467.33+/-0.43) cm(-1) and (390.77+/-0.56) cm(-1).

“…Fortunately, both algorithmic advances [8][9][10][11][12][13][14][15] and improvements in computer hardware have made full CI benchmarks less computationally expensive. Whereas full CI benchmarking in the 1980s and early 1990s focused almost exclusively on single-point energies, it is now possible to perform geometry optimizations and even frequency analysis for very small molecules using full CI to examine the effect of higher-order correlation on molecular properties.…”

Section: Introductionmentioning

confidence: 99%

A comparison of polarized double-zeta basis sets and natural orbitals for full configuration interaction benchmarks

Abrams

Sherrill

2003

We compare several standard polarized double-zeta basis sets for use in full configuration interaction benchmark computations. The 6-31G**, DZP, cc-pVDZ, and Widmark-Malmqvist-Roos atomic natural orbital ͑ANO͒ basis sets are assessed on the basis of their ability to provide accurate full configuration interaction spectroscopic constants for several small molecules. Even though highly correlated methods work best with larger basis sets, predicted spectroscopic constants are in good agreement with experiment; bond lengths and harmonic vibrational frequencies have average absolute errors no larger than 0.017 Å and 1.6%, respectively, for all but the ANO basis. For the molecules considered, 6-31G** gives the smallest average errors, while the ANO basis set gives the largest. The use of variationally optimized basis sets and natural orbitals are also explored for improved benchmarking. Although optimized basis sets do not always improve predictions of molecular properties, taking a DZP-sized subset of the natural orbitals from a singles and doubles configuration interaction computation in a larger basis significantly improves results.