65 research outputs found
A Cholesky decomposition-based implementation of relativistic two-component coupled-cluster methods for medium-sized molecules
A Cholesky decomposition (CD)-based implementation of relativistic
two-component coupled-cluster (CC) and equation-of-motion CC (EOM-CC) methods
using an exact two-component Hamiltonian augmented with atomic-mean-field
spin-orbit integrals (the X2CAMF scheme) is reported. The present CD-based
implementation of X2CAMF-CC and EOM-CC methods employs atomic-orbital-based
algorithms to avoid the construction of two-electron integrals and
intermediates involving three and four virtual indices. Our CD-based
implementation extends the applicability of X2CAMF-CC and EOM-CC methods to
medium-sized molecules with the possibility to correlate around 1000 spinors.
Benchmark calculations for uranium-containing small molecules have been
performed to assess the dependence of the CC results on the Cholesky threshold.
A Cholesky threshold of is shown to be sufficient to maintain
chemical accuracy. Example calculations to illustrate the capability of the
CD-based relativistic CC methods are reported for the bond-dissociation energy
of the uranium hexafluoride molecule, UF, with up to quadruple-zeta basis
sets, and the lowest excitation energy in solvated uranyl ion
[UO(HO)]
GW quasiparticle energies of atoms in strong magnetic fields
Quasiparticle energies of the atoms HâNe have been computed in the GW approximation in the presence of strong magnetic fields with field strengths varying from 0 to 0.25 atomic units (0.25âB 0 =0.25ââe â1 a â2 0 â58â763 0.25âB0=0.25ââeâ1a0â2â58â763 T). The GW quasiparticle energies are compared with equation-of-motion ionization-potential (EOM-IP) coupled-cluster singles-and-doubles (CCSD) calculations of the first ionization energies. The best results are obtained with the evGW@PBE0 method, which agrees with the EOM-IP-CCSD model to within about 0.20 eV. Ionization potentials have been calculated for all atoms in the series, representing the first systematic study of ionization potentials for the first-row atoms at field strengths characteristic of magnetic white dwarf stars. Under these conditions, the ionization potentials increase in a near-linear fashion with the field strength, reflecting the linear field dependence of the Landau energy of the ionized electron. The calculated ionization potentials agree well with the best available literature data for He, Li, and Be
Current density functional theory using meta-generalized gradient exchange-correlation functionals
We present the self-consistent implementation of current-dependent (hybrid)
meta generalized gradient approximation (mGGA) density functionals using London
atomic orbitals. A previously proposed generalized kinetic energy density is
utilized to implement mGGAs in the framework of Kohn--Sham current
density-functional theory (KS-CDFT). A unique feature of the non-perturbative
implementation of these functionals is the ability to seamlessly explore a wide
range of magnetic fields up to 1 a.u. (T) in strength. CDFT
functionals based on the TPSS and B98 forms are investigated and their
performance is assessed by comparison with accurate CCSD(T) data. In the weak
field regime magnetic properties such as magnetizabilities and NMR shielding
constants show modest but systematic improvements over GGA functionals.
However, in strong field regime the mGGA based forms lead to a significantly
improved description of the recently proposed perpendicular paramagnetic
bonding mechanism, comparing well with CCSD(T) data. In contrast to functionals
based on the vorticity these forms are found to be numerically stable and their
accuracy at high field suggests the extension of mGGAs to CDFT via the
generalized kinetic energy density should provide a useful starting point for
further development of CDFT approximations
The hyperfine structure in the rotational spectra of bromofluoromethane: Lamb-dip technique and quantum-chemical calculations
International audienceThe hyperfine structure in the rotational spectra of six isotopic species of bromofluoromethane, namely CH2{79}BrF, CH2{81}BrF, CDH{79}BrF, CDH{81}BrF, CD2{79}BrF, and CD2{81}BrF, has been investigated using the Lamb-dip technique in the submillimeter-wave frequency range. Measurements as well as assignment procedures have been supported by high-level quantum-chemical calculations of the hyperfine parameters at the coupled-cluster level. For all species, the accuracy of the determined rotational and centrifugal distortion constants as well as the bromine quadrupole-coupling constants have been improved with respect to available literature data, whereas the full bromine quadrupole-coupling tensor for the monodeuterated species and the bromine spin-rotation constants for all isotopologues have been determined for the first time
Coupled-cluster techniques for computational chemistry: The CFOUR program package
An up-to-date overview of the CFOUR program system is given. After providing a brief outline of the evolution of the program since its inception in 1989, a comprehensive presentation is given of its well-known capabilities for high-level coupled-cluster theory and its application to molecular properties. Subsequent to this generally well-known background information, much of the remaining content focuses on lesser-known capabilities of CFOUR, most of which have become available to the public only recently or will become available in the near future. Each of these new features is illustrated by a representative example, with additional discussion targeted to educating users as to classes of applications that are now enabled by these capabilities. Finally, some speculation about future directions is given, and the mode of distribution and support for CFOUR are outlined
A DZ white dwarf with a 30âMG magnetic field
Magnetic white dwarfs with field strengths below 10âMG are easy to recognize since the Zeeman splitting of spectral lines appears proportional to the magnetic field strength. For fields âł100âMG, however, transition wavelengths become chaotic, requiring quantum-chemical predictions of wavelengths and oscillator strengths with a non-perturbative treatment of the magnetic field. While highly accurate calculations have previously been performed for hydrogen and helium, the variational techniques employed become computationally intractable for systems with more than three to four electrons. Modern computational techniques, such as finite-field coupled-cluster theory, allow the calculation of many-electron systems in arbitrarily strong magnetic fields. Because around 25 perâcent of white dwarfs have metal lines in their spectra, and some of those are also magnetic, the possibility arises for some metals to be observed in very strong magnetic fields, resulting in unrecognizable spectra. We have identified SDSSâJ114333.48+661531.83 as a magnetic DZ white dwarf, with a spectrum exhibiting many unusually shaped lines at unknown wavelengths. Using atomic data calculated from computational finite-field coupled-cluster methods, we have identified some of these lines arising from Na, Mg, and Ca. Surprisingly, we find a relatively low field strength of 30âMG, where the large number of overlapping lines from different elements make the spectrum challenging to interpret at a much lower field strength than for DAs and DBs. Finally, we model the field structure of SDSSâJ1143+6615 finding the data are consistent with an offset dipole
KohnâSham energy decomposition for molecules in a magnetic field
We study the total molecular electronic energy and its KohnâSham components within the framework of magnetic-field density-functional theory (BDFT), an alternative to current-dependent density-functional theory (CDFT) for molecules in the presence of magnetic fields. For a selection of closed-shell dia- and paramagnetic molecules, we investigate the dependence of the total electronic energy and its KohnâSham components on the magnetic field. Results obtained from commonly used density-functional approximations are compared with those obtained from Lieb optimizations based on magnetic-field dependent relaxed coupled-cluster singles-and-doubles (CCSD) and second-order MĂžllerâPlesset (MP2) densities. We show that popular approximate exchangeâcorrelation functionals at the generalized-gradient-approximation (GGA), meta-GGA, and hybrid levels of theory provide a good qualitative description of the electronic energy and its KohnâSham components in a magnetic fieldâin particular, for the diamagnetic molecules. The performance of HartreeâFock theory, MP2 theory, CCSD theory and BDFT with different exchangeâcorrelation functionals is compared with coupled-cluster singles-doubles-perturbative-triples (CCSD(T)) theory for the perpendicular component of the magnetizability. Generalizations of the TPSS meta-GGA functional to systems in a magnetic field work wellâthe cTPSS functional, in particular, with a current-corrected kinetic-energy density, performs excellently, providing an accurate and balanced treatment of dia- and paramagnetic systems and outperforming MP2 theory
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