9,058 research outputs found
Approaching exact hyperpolarizabilities via sum-over-states Monte Carlo configuration interaction
We propose using sum-over-states calculations with the compact wavefunctions
of Monte Carlo configuration interaction to approach accurate values for
higher-order dipole properties up to second hyperpolarizabilities in a
controlled way. We apply the approach to small systems that can generally be
compared with full configuration interaction (FCI) results. We consider
hydrogen fluoride with a 6-31g basis and then look at results, including
frequency dependent properties, in an aug-cc-pVDZ basis. We extend one
calculation beyond FCI by using an aug-cc-pVTZ basis. The properties of an
H molecule with multireference character are calculated in an aug-cc-pVDZ
basis. We then investigate this method on a strongly multireference system with
a larger FCI space by modelling the properties of carbon monoxide with a
stretched geometry. The behavior of the approach with increasing basis size is
considered by calculating results for the neon atom using aug-cc-pVDZ to
aug-cc-pVQZ. We finally test if the unusual change in polarizability between
the first two states of molecular oxygen can be reproduced by this method in a
6-31g basis.Comment: 11 pages, 14 figure
Characterizing a configuration interaction excited state using natural transition geminals
We introduce natural transition geminals as a means to qualitatively
understand a transition where double excitations are important. The first two
singlet states of the CH cation are used as an initial example. We
calculate these states with configuration interaction singles (CIS) and
state-averaged Monte Carlo configuration interaction (SA-MCCI). For each method
we compare the important natural transition geminals with the dominant natural
transition orbitals. We then compare SA-MCCI and full configuration interaction
(FCI) with regards to the natural transition geminals using the beryllium atom.
We compare using the natural transition geminals with analyzing the important
configurations in the CI expansion to give the dominant transition for the
beryllium atom and the carbon dimer. Finally we calculate the natural
transition geminals for two electronic excitations of formamide.Comment: 15 pages, 2 figures. Mol. Phys. (2013
Applying Monte Carlo configuration interaction to transition metal dimers: exploring the balance between static and dynamic correlation
We calculate potential curves for transition metal dimers using Monte Carlo
configuration interaction (MCCI). These results, and their associated
spectroscopic values, are compared with experimental and computational studies.
The multireference nature of the MCCI wavefunction is quantified and we
estimate the important orbitals. We initially consider the ground state of the
chromium dimer. Next we calculate potential curves for Sc where we
contrast the lowest triplet and quintet states. We look at the molybdenum dimer
where we compare non-relativistic results with the partial inclusion of
relativistic effects via effective core potentials, and report results for
scandium nickel.Comment: 9 pages and 8 figure
Calculations of Potential Energy Surfaces Using Monte Carlo Configuration Interaction
We apply the method of Monte Carlo configuration interaction (MCCI) to
calculate ground-state potential energy curves for a range of small molecules
and compare the results with full configuration interaction. We show that the
MCCI potential energy curve can be calculated to relatively good accuracy, as
quantified using the non-parallelity error, using only a very small fraction of
the FCI space. In most cases the potential curve is of better accuracy than its
constituent single-point energies. We finally test the MCCI program on systems
with basis sets beyond full configuration interaction: a lattice of fifty
hydrogen atoms and ethylene. The results for ethylene agree fairly well with
other computational work while for the lattice of fifty hydrogens we find that
the fraction of the full configuration interaction space we were able to
consider appears to be too small as, although some qualitative features are
reproduced, the potential curve is less accurate.Comment: 14 pages, 22 figure
Monte Carlo configuration interaction applied to multipole moments, ionisation energies and electron affinities
The method of Monte Carlo configuration interaction (MCCI) [1,2] is applied
to the calculation of multipole moments. We look at the ground and excited
state dipole moments in carbon monoxide. We then consider the dipole of NO, the
quadrupole of the nitrogen molecule and of BH. An octupole of methane is also
calculated. We consider experimental geometries and also stretched bonds. We
show that these non-variational quantities may be found to relatively good
accuracy when compared with FCI results, yet using only a small fraction of the
full configuration interaction space. MCCI results in the aug-cc-pVDZ basis are
seen to generally have reasonably good agreement with experiment. We also
investigate the performance of MCCI when applied to ionisation energies and
electron affinities of atoms in an aug-cc-pVQZ basis. We compare the MCCI
results with full configuration-interaction quantum Monte Carlo [3,4] and
`exact' non-relativistic results [3,4]. We show that MCCI could be a useful
alternative for the calculation of atomic ionisation energies however electron
affinities appear much more challenging for MCCI. Due to the small magnitude of
the electron affinities their percentage errors can be high, but with regards
to absolute errors MCCI performs similarly for ionisation energies and electron
affinities.Comment: 12 pages, 20 figure
A comparative study of benchmarking approaches for non-domestic buildings: Part 1 – Top-down approach
Benchmarking plays an important role in improving energy efficiency of non-domestic buildings. A review of energy benchmarks that underpin the UK’s Display Energy Certificate (DEC) scheme have prompted necessities to explore the benefits and limitations of using various methods to derive energy benchmarks. The existing methods were reviewed and grouped into top-down and bottom-up approaches based on the granularity of the data used. In the study, two top-down methods, descriptive statistics and artificial neural networks (ANN), were explored for the purpose of benchmarking energy performances of schools. The results were used to understand the benefits of using these benchmarks for assessing energy efficiency of buildings and the limitations that affect the robustness of the derived benchmarks. Compared to the bottom-up approach, top-down approaches were found to be beneficial in gaining insight into how peers perform. The relative rather than absolute feedback on energy efficiency meant that peer pressure was a motivator for improvement. On the other hand, there were limitations with regard to the extent to which the energy efficiency of a building could be accurately assessed using the top-down benchmarks. Moreover, difficulties in acquiring adequate data were identified as a key limitation to using the top-down approach for benchmarking non-domestic buildings. The study suggested that there are benefits in rolling out of DECs to private sector buildings and that there is a need to explore more complex methods to provide more accurate indication of energy efficiency in non-domestic buildings
Gallium oxide and gadolinium gallium oxide insulators on Si δ-doped GaAs/AlGaAs heterostructures
Test devices have been fabricated on two specially grown GaAs/AlGaAs wafers with 10 nm thick gate dielectrics composed of either Ga<sub>2</sub>O<sub>3</sub> or a stack of Ga<sub>2</sub>O<sub>3</sub> and Gd<sub>0.25</sub>Ga<sub>0.15</sub>O<sub>0.6</sub>. The wafers have two GaAs transport channels either side of an AlGaAs barrier containing a Si delta-doping layer. Temperature dependent capacitance-voltage (C-V) and current-voltage (I-V) studies have been performed at temperatures between 10 and 300 K. Bias cooling experiments reveal the presence of DX centers in both wafers. Both wafers show a forward bias gate leakage that is by a single activated channel at higher temperatures and by tunneling at lower temperatures. When Gd<sub>0.25</sub>Ga<sub>0.15</sub>O<sub>0.6</sub> is included in a stack with 1 nm of Ga<sub>2</sub>O<sub>3</sub> at the interface, the gate leakage is greatly reduced due to the larger band gap of the Gd<sub>0.25</sub>Ga<sub>0.15</sub>O<sub>0.6</sub> layer. The different band gaps of the two oxides result in a difference in the gate voltage at the onset of leakage of ~3 V. However, the inclusion of Gd<sub>0.25</sub>Ga<sub>0.15</sub>O<sub>0.6</sub> in the gate insulator introduces many oxide states (≤4.70Ã�Â�10<sup>12</sup> cm<sup>âÂ�Â�2</sup>). Transmission electron microscope images of the interface region show that the growth of a Gd<sub>0.25</sub>Ga<sub>0.15</sub>O<sub>0.6</sub> layer on Ga<sub>2</sub>O<sub>3</sub> disturbs the well ordered Ga<sub>2</sub>O<sub>3</sub>/GaAs interface. We therefore conclude that while including Gd<sub>0.25</sub>Ga<sub>0.15</sub>O<sub>0.6</sub> in a dielectric stack with Ga<sub>2</sub>O<sub>3</sub> is necessary for use in device applications, the inclusion of Gd decreases the quality of the Ga<sub>2</sub>O<sub>3</sub>/GaAs interface and near interface region by introducing roughness and a large number of defect states
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