ABSTRACT
We devise an efficient scheme to determine vibrational properties from Path Integral Molecular Dynamics (PIMD) simulations. The method is based on zero-time Kubo-transformed correlation functions and captures the anharmonicity of the potential due to both temperature and quantum effects. Using analytical derivations and numerical calculations on toy-model potentials, we show that two different estimators built upon PIMD correlation functions fully characterize the phonon spectra and the anharmonicity strength. The first estimator is associated with the force-force quantum correlators and, in the weak anharmonic regime, yields reliable zero-point motion frequencies and thermodynamic properties of the quantum system. The second one is instead connected to displacement-displacement correlators and accurately probes the lowest-energy phonon excitations, regardless of the anharmonicity strength of the system. We also prove that the use of generalized eigenvalue equations, in place of the standard normal mode equations, leads to a significant speed-up in the PIMD phonon calculations, both in terms of faster convergence rate and smaller time step bias. Within this framework, using ab initio PIMD simulations, we compute phonon dispersions of diamond and of the high-pressure I41/amd phase of atomic hydrogen. We find that in the latter case, the anharmonicity is stronger than previously estimated and yields a sizeable red-shift in the vibrational spectrum of atomic hydrogen.
ABSTRACT
Quantum EXPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the-art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudopotential and projector-augmented-wave approaches. Quantum EXPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement their ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.
ABSTRACT
The mechanism of photomagnetism in copper octacyanomolybdate molecules is currently under debate. Contrary to the general belief that the photomagnetic transition occurs only due to a photoinduced electron transfer from the molybdenum to the copper atom, recent X-ray magnetic dichroic (XMCD) data clearly indicate that this phenomenon is associated at low temperature to a local low-spin-high-spin transition on the molybdenum atom. In this article we provide theoretical justification for these experimental facts. We show the first simulation of X-ray absorption (XAS) and magnetic circular dichroism (XMCD) spectra at the L(2,3) edges of molybdenum from the joint perspective of density functional theory (DFT) calculations and ligand field multiplet (LFM) theory. The description of electronic interactions seems mandatory for reproducing the photomagnetic state.
