arXiv subjects

Martin A. Mosquera

Publications and source records attributed to Martin A. Mosquera.

7 recordsLinked to original sources

arXiv2026

Observables and Anti-Hermitian Generators in Time-Dependent Unitary Coupled Cluster Theory

This work presents a time-dependent (TD) unitary coupled-cluster (UCC) formulation for electronic quantum dynamics, including the propagation of excited states and their superpositions, in both single- and multi-reference regimes. Standard TD coupled-cluster techniques offer size-extensivity, but they rely on non-Hermitian bivariational action functionals that break time-reversibility, giving transition matrix elements and amplitude estimators that are asymmetric (though accurate and systematically improvable). Here we use the time-evolution operator as an exponential map driven by TD anti-Hermitian cluster operators and first-order generators. Applying the Dirac-Frenkel action principle, we extract equations of motion governed by Heisenberg-picture-like commutators. This approach connects to our previous non-Hermitian formulations, where observables are expressed in terms of regular and extended cluster operators. From that connection we obtain a generator cluster operator whose unperturbed TD limit leads to the UCC eigenvalue problem. Even though this problem can be exact, the time dependence of the generator holds only at short propagation times, so we use the generator to prepare the initial state and then propagate that state with the formal TD UCC equation of motion. The theory is tested on an extended hard-core Bose-Hubbard ring with connections to neutral atom chains, and we discuss the theory's present limitations and possible extensions.

physics.chem-ph↗

arXiv2022

Slippery paraelectric transition metal dichalcogenide bilayers

Traditional ferroelectrics undergo thermally-induced phase transitions whereby their structural symmetry increases. The associated higher-symmetry structure is dubbed {\em paraelectric}. Ferroelectric transition metal dichalcogenide bilayers have been recently shown to become paraelectric, but not much has been said of the atomistic configuration of such a phase. As discovered through numerical calculations that include molecular dynamics here, their paraelectricity can only be ascribed to a time average of ferroelectric phases with opposing intrinsic polarizations, whose switching requires macroscopically large areas to slip in unison.

cond-mat.mtrl-sci↗

arXiv2022

Site-specific surface atom valence band structure via X-ray standing wave excited photoemission

X-ray standing wave (XSW) excited photoelectron emission was used to measure the site-specific valence band (VB) for 1/2 monolayer (ML) Pt grown on a SrTiO3 (001) surface. The XSW induced modulations in the core level (CL) and VB photoemission from the surface and substrate atoms were monitored for three hkl substrate Bragg reflections. The XSW CL analysis shows the Pt to have an fcc-like cube-on-cube epitaxy with the substrate. The XSW VB information compares well to a density functional theory calculated projected density of states from the surface and substrate atoms. Overall, this work represents a novel method for determining the contribution to the density of states by valence electrons from specific atomic surface sites.

cond-mat.mtrl-sci↗

arXiv2014

Integer Discontinuity of Density Functional Theory

Density functional approximations to the exchange-correlation energy of Kohn-Sham theory, such as the local density approximation and generalized gradient approximations, lack the well-known integer discontinuity, a feature that is critical to describe molecular dissociation correctly. Moreover, standard approximations to the exchange-correlation energy also fail to yield the correct linear dependence of the ground-state energy on the number of electrons when this is a non-integer number obtained from the grand canonical ensemble statistics. We present a formal framework to restore the integer discontinuity of any density functional approximation. Our formalism derives from a formula for the exact energy functional and a new constrained search functional that recovers the linear dependence of the energy on the number of electrons.

physics.chem-ph↗

arXiv2013

On the Action Formalism of Time-dependent Density-functional Theory

The Runge-Gross [E. Runge, and E. K. U. Gross, Phys. Rev. Lett., 52, 997 (1984)] action functional of time-dependent density-functional theory leads to a well-known causality paradox, i.e., a perturbation of the electronic density in the future affects the response of the system in the present. This paradox is known to be caused by an inconsistent application of the Dirac-Frenkel variational principle. In view of the recent solutions to this problem, the action functional employed by Runge and Gross in their formulation of time-dependent density functional theory is analyzed in the context of the Keldysh contour technique. The time-dependent electronic density, as well as the concept of causality, are extended to the contour. We derive a variational equation that obeys causality and relates the exchange-correlation potential with its kernel, and the functional derivative of the exchange-correlation action functional with respect to the density. It is shown that the adiabatic local-density approximation is a consistent solution of this equation and that the time-dependent optimized potential method can also be derived from it. The formalism presented here can be used to find new approximations methods to the exchange-correlation potential and avoid the causality dilemma.

physics.chem-ph↗

arXiv2013

Fragment-based Time-dependent Density-functional Theory

Using the Runge-Gross theorem that establishes the foundation of Time-dependent Density Functional Theory (TDDFT) we prove that for a given electronic Hamiltonian, choice of initial state, and choice of fragmentation, there is a unique single-particle potential (dubbed time-dependent partition potential) which, when added to each of the pre-selected fragment potentials, forces the fragment densities to evolve in such a way that their sum equals the exact molecular density at all times. This uniqueness theorem suggests new ways of computing time-dependent properties of electronic systems via fragment-TDDFT calculations. We derive a formally exact relationship between the partition potential and the total density, and illustrate our approach on a simple model system for binary fragmentation in a laser field.

physics.chem-ph↗

arXiv2012

Simple isotherm equations to fit type I adsorption data

A simple model to fit experimental data of adsorption of gases and vapours on microporous adsorbents (type I isotherms) is proposed. The main assumption is that the adsorbate phase can be divided into identical and non-interacting effective subsystems. This gives rise to a simple multiparametric isotherm based on the grand canonical ensemble statistics, whose functional form is a ratio of two polynomial functions. The parameters are interpreted as effective equilibrium constants. A simplified isotherm that reduces the number of adjustable parameters with respect to the general isotherm is also proposed. We show how to use these isotherms to fit the adsorption data in such way that the parameters have statistical significance. Due to their high accuracy, both isotherms can be used to estimate thermodynamic properties like isosteric and differential heats of adsorption. A simple method is presented for systems that show an apparent variation in the coverage limit with temperature. This method avoids overparametrization and improves fitting deviations. Finally, several applications to fitting data, taken from literature, of adsorption of some gases on activated carbon, molecular sieving carbon, silica gel, and pillared clays are presented.

physics.chem-ph↗