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L Mitas

Publications and source records attributed to L Mitas.

7 recordsLinked to original sources
PubMed2006

Pfaffian pairing wave functions in electronic-structure quantum Monte Carlo simulations.

We investigate the accuracy of trial wave functions for quantum Monte Carlo based on Pfaffian functional form with singlet and triplet pairing. Using a set of first row atoms and molecules we find that these wave functions provide very consistent and systematic behavior in recovering the correlation energies on the level of 95%. In order to get beyond this limit we explore the possibilities of multi-Pfaffian pairing wave functions. We show that a small number of Pfaffians recovers another large fraction of the missing correlation energy comparable to the larger-scale configuration interaction wave functions. We also find that Pfaffians lead to substantial improvements in fermion nodes when compared to Hartree-Fock wave functions.

Journal Article↗
PubMed2001

High accuracy many-body calculational approaches for excitations in molecules.

Two state-of-the-art computational approaches: quantum Monte Carlo and GW with exciton effects [GW-BSE (Bethe-Salpeter equation)] are employed to calculate ionization potentials, electron affinities, and first excited singlet and triplet energies for the silane and methane molecules. Results are in excellent agreement between these dramatically different approaches and with available experiment. The optically forbidden triplet excitation in silane is predicted to lie roughly 1 eV higher than previously reported. In the GW-BSE method, we demonstrate that inclusion of off-diagonal matrix elements in the self-energy operator is crucial for an accurate picture.

Journal Article↗
PubMed2000

Electron correlation in C(4N+2) carbon rings: aromatic versus dimerized structures

The electronic structure of C(4N+2) carbon rings exhibits competing many-body effects of Huckel aromaticity, second-order Jahn-Teller (SOJT), and Peierls instability at large sizes. This leads to possible ground state structures with aromatic, bond angle, or bond length alternated geometry. Highly accurate quantum Monte Carlo results indicate the existence of a crossover between C10 and C14 from bond angle to bond length alternation. The aromatic isomer is always a transition state. The gap opening mechanism is the SOJT effect, which coalesces with the Peierls regime as N-->infinity.

Journal Article↗
PubMed2000

Silicon clusters of intermediate size: energetics, dynamics, and thermal effects

A combination of the best available theoretical techniques for energetics, dynamics, and thermodynamics is employed in an extensive study of Si(n) ( n = 20,25) clusters. For T = 0 we solve the electronic structure by the density functional and the highly accurate quantum Monte Carlo approaches. Finite temperature and dynamical effects are investigated by the ab initio molecular dynamics method. This combination of methods enables us to find several new low-energy isomers and to explain the differences in properties, behavior, and stability of elongated versus compact types of structures and to elucidate the origin of the existing discrepancies between theory and experiments.

Journal Article↗
PubMed1989

[Evaluation of the clinical status of newborn infants born to elderly primiparae and multiparae].

The clinical condition of 184 newborns born by primipara older than 35 years and 500 randomisly selected newborns born by multipara older than 35 years in 27,821 newborns born at 1st Department of Obstetrics and Gynecology in Bytom between 1975 and 1985 has been analysed. The results were compared with the results of the control group of 500 randomisly selected newborns born by primipara younger than 35 years. It was observed that newborns of mothers older than 35 years and specially primipara are biologically weaker than newborns of mothers of the optimal reproduction age. There are often born with signs of prematurity and intrauterine dystrophy, have lower body weight, higher incidence of developmental anomaly, labour complications and adaptation difficulties during early neonatal period. The importance was stressed of the monitoring of pregnancy and delivery in this patients as well as the intensive perinatal pharmacotherapy in the newborns, to decrease perinatal mortality.

Adult↗
PubMedDate not supplied

[Ormond's disease: difficulties with diagnosis and treatment].

A case of a female patient is presented. The patient was treated for retroperitoneal fasciitis. An emphasis is on the diagnostic, differential, and therapeutic problems in this entity. Close cooperation of various specialists is needed in such a case.

Female↗