arXiv subjects

Rustam Z. Khaliullin

Publications and source records attributed to Rustam Z. Khaliullin.

14 recordsLinked to original sources

arXiv2026

CP2K: An electronic structure and molecular dynamics software package - Dynamics, Transport, and Spectroscopic Response

One of the distinguishing aspects of CP2K is its seamless integration of diverse structural and transition-state optimization techniques with advanced sampling approaches including Monte Carlo, molecular dynamics, and metadynamics, enabling the efficient exploration of complex potential- and free-energy landscapes, including rare events. These capabilities are combined with a broad hierarchy of energy and force evaluation methods, ranging from classical and machine-learned interaction potentials and mixed quantum-classical multiscale and semiempirical schemes, to highly accurate quantum-mechanical electronic-structure approaches. At the heart of the latter lies the Gaussian and plane-wave framework, along with its augmented all-electron generalization, which have been described in detail in our previous code review [T. D. Kühne et al., J. Chem. Phys. 152, 194103 (2020)]. Building on this foundation, the present work revisits the methods within CP2K that turn electronic structure into dynamics, transport, and spectroscopic response. Particular emphasis is placed on the coupling between static response calculations and nuclear motion: spectra may be evaluated at optimized structures, averaged over thermally sampled configurations, obtained from time-correlation functions along ab-initio or path integral molecular trajectories, or followed in real time together with electronic and nuclear dynamics. The same modular structure also enables equilibrium and biased transport simulations, from Kubo-type linear response to open-boundary approaches under external potentials, highlighting CP2K's unique capability to unify quantum chemistry with quantum and statistical mechanics within a versatile, holistic simulation environment.

physics.chem-ph↗

arXiv2025

The CP2K Program Package Made Simple

CP2K is a versatile open-source software package for simulations across a wide range of atomistic systems, from isolated molecules in the gas phase to low-dimensional functional materials and interfaces, as well as highly symmetric crystalline solids, disordered amorphous glasses, and weakly interacting soft-matter systems in the liquid state and in solution. This review highlights CP2K's capabilities for computing both static and dynamical properties using quantum-mechanical and classical simulation methods. In contrast to the accompanying theory and code paper [J. Chem. Phys. 152, 194103 (2020)], the focus here is on the practical usage and applications of CP2K, with underlying theoretical concepts introduced only as needed.

physics.comp-ph↗

arXiv2025

Direct unconstrained optimization of excited states in density functional theory

Orbital-optimized density functional theory (DFT) has emerged as an alternative to time-dependent (TD) DFT capable of describing difficult excited states with significant electron density redistribution, such as charge-transfer, Rydberg, and double-electron excitations. Here, a simple method is developed to solve the main problem of the excited-state optimization -- the variational collapse of the excited states onto the ground state. In this method, called variable-metric time-independent DFT (VM TIDFT), the electronic states are allowed to be nonorthogonal during the optimization but their orthogonality is gradually enforced with a continuous penalty function. With nonorthogonal electronic states, VM TIDFT can use molecular orbital coefficients as independent variables, which results in a closed-form analytical expression for the gradient and allows to employ any of the multiple unconstrained optimization algorithms that guarantees convergence of the excited-state optimization. Numerical tests on multiple molecular systems show that the variable-metric optimization of excited states performed with a preconditioned conjugate gradient algorithm is robust and produces accurate energies for well-behaved excitations and, unlike TDDFT, for more challenging charge-transfer and double-electron excitations.

physics.chem-ph↗

arXiv2021

Robust linear-scaling optimization of compact localized orbitals in density functional theory

Locality of compact one-electron orbitals expanded strictly in terms of local subsets of basis functions can be exploited in density functional theory (DFT) to achieve linear growth of computation time with systems size, crucial in large-scale simulations. However, despite advantages of compact orbitals the development of practical orbital-based linear-scaling DFT methods has long been hindered because a compact representation of the electronic ground state is difficult to find in a variational optimization procedure. In this work, we show that the slow and unstable optimization of compact orbitals originates from the nearly-invariant mixing of compact orbitals that are mostly but not completely localized within the same subsets of basis functions. We also construct an approximate Hessian that can be used to identify the problematic nearly-invariant modes and obviate the variational optimization along them without introducing significant errors into the computed energies. This enables us to create a linear-scaling DFT method with a low computational overhead that is demonstrated to be efficient and accurate in fixed-nuclei calculations and molecular dynamics simulations of semiconductors and insulators.

physics.comp-ph↗

arXiv2020

CP2K: An Electronic Structure and Molecular Dynamics Software Package -- Quickstep: Efficient and Accurate Electronic Structure Calculations

CP2K is an open source electronic structure and molecular dynamics software package to perform atomistic simulations of solid-state, liquid, molecular and biological systems. It is especially aimed at massively-parallel and linear-scaling electronic structure methods and state-of-the-art ab-initio molecular dynamics simulations. Excellent performance for electronic structure calculations is achieved using novel algorithms implemented for modern high-performance computing systems. This review revisits the main capabilities of CP2k to perform efficient and accurate electronic structure simulations. The emphasis is put on density functional theory and multiple post-Hartree-Fock methods using the Gaussian and plane wave approach and its augmented all-electron extension.

physics.chem-ph↗

arXiv2020

Direct unconstrained variable-metric localization of one-electron orbitals

Spatially localized one-electron orbitals, orthogonal and nonorthogonal, are widely used in electronic structure theory to describe chemical bonding and speed up calculations. In order to avoid linear dependencies of localized orbitals, the existing localization methods either constrain orbital transformations to be unitary, that is, metric preserving, or, in the case of variable-metric methods, fix the centers of nonorthogonal localized orbitals. Here, we developed a different approach to orbital localization, in which these constraints are replaced with a single restriction that specifies the maximum allowed deviation from the orthogonality for the final set of localized orbitals. This reformulation, which can be viewed as a generalization of existing localization methods, enables one to choose the desired balance between the orthogonality and locality of the orbitals. Furthermore, the approach is conceptually and practically simple as it obviates the necessity in unitary transformations and allows to determine optimal positions of the centers of nonorthogonal orbitals in a unconstrained and straightforward minimization procedure. It is demonstrated to produce well-localized orthogonal and nonorthogonal orbitals with the Berghold and Pipek-Mezey localization functions for a variety of molecules and periodic materials including large systems with non-trivial bonding.

physics.chem-ph↗

arXiv2018

Low-cost orbital-based linear-scaling \emph{ab initio} molecular dynamics for weakly-interacting systems

Within the framework of linear-scaling Kohn-Sham density functional theory, a robust method for maintaining compact localized orbitals close to the ground state is coupled with nuclear dynamics. This allows to obviate the commonly employed optimization of the one-electron density matrix and thus create an efficient orbital-only molecular dynamics method for weakly-interacting systems. An application to liquid water demonstrates that the low computational overhead of the method makes it well-suited for routine simulations whereas its linear-scaling complexity allows to extend first-principle dynamical studies of molecular systems to previously inaccessible length scales.

physics.comp-ph↗

arXiv2014

On the role of interfacial hydrogen bonds in "on-water" catalysis

Numerous experiments have demonstrated that many classes of organic reactions exhibit increased reaction rates when performed in heterogeneous water emulsions. Despite enormous practical importance of the observed "on-water" catalytic effect and several mechanistic studies, its microscopic origins remains unclear. In this work, the second generation Car-Parrinello molecular dynamics method is extended to self-consistent charge density-functional based tight-binding in order to study "on-water" catalysis of the Diels-Alder reaction between dimethyl azodicarboxylate and quadricyclane. We find that the stabilization of the transition state by dangling hydrogen bonds exposed at the aqueous interfaces plays a significantly smaller role in "on-water" catalysis than has been suggested previously.

physics.chem-ph↗

arXiv2013

Recent achievements in ab initio modelling of liquid water

The application of newly developed first-principle modeling techniques to liquid water deepens our understanding of the microscopic origins of its unusual macroscopic properties and behaviour. Here, we review two novel ab initio computational methods: second-generation Car-Parrinello molecular dynamics and decomposition analysis based on absolutely localized molecular orbitals. We show that these two methods in combination not only enable ab initio molecular dynamics simulations on previously inaccessible time and length scales, but also provide unprecedented insights into the nature of hydrogen bonding between water molecules. We discuss recent applications of these methods to water clusters and bulk water.

physics.chem-ph↗

arXiv2013

Electronic signature of the instantaneous asymmetry in the first coordination shell of liquid water

Interpretation of the X-ray spectra of water as evidence for its asymmetric structure has challenged the conventional symmetric nearly-tetrahedral model and initiated an intense debate about the order and symmetry of the hydrogen bond network in water. Here, we present new insights into the nature of local interactions in water obtained using a novel energy decomposition method. Our simulations reveal that while a water molecule forms, on average, two strong donor and two strong acceptor bonds, there is a significant asymmetry in the energy of these contacts. We demonstrate that this asymmetry is a result of small instantaneous distortions of hydrogen bonds, which appear as fluctuations on a timescale of hundreds of femtoseconds around the average symmetric structure. Furthermore, we show that the distinct features of the X-ray absorption spectra originate from molecules with high instantaneous asymmetry. Our findings have important implications as they help reconcile the symmetric and asymmetric views on the structure of water.

physics.chem-ph↗

arXiv2011

Microscopic origins of the anomalous melting behaviour of high-pressure sodium

Recent experiments have shown that sodium, a prototype simple metal at ambient conditions, exhibits unexpected complexity under high pressure. One of the most puzzling phenomena in the behaviour of dense sodium is the pressure-induced drop in its melting temperature, which extends from 1000 K at ~30GPa to as low as room temperature at ~120GPa. Despite significant theoretical effort to understand the anomalous melting its origins have remained unclear. In this work, we reconstruct the sodium phase diagram using an ab-initio-quality neural-network potential. We demonstrate that the reentrant behaviour results from the screening of interionic interactions by conduction electrons, which at high pressure induces a softening in the short-range repulsion. It is expected that such an effect plays an important role in governing the behaviour of a wide range of metals and alloys.

cond-mat.mtrl-sci↗

arXiv2011

Nucleation mechanism for the direct graphite-to-diamond phase transition

Graphite and diamond have comparable free energies, yet forming diamond from graphite is far from easy. In the absence of a catalyst, pressures that are significantly higher than the equilibrium coexistence pressures are required to induce the graphite-to-diamond transition. Furthermore, the formation of the metastable hexagonal polymorph of diamond instead of the more stable cubic diamond is favored at lower temperatures. The concerted mechanism suggested in previous theoretical studies cannot explain these phenomena. Using an ab initio quality neural-network potential we performed a large-scale study of the graphite-to-diamond transition assuming that it occurs via nucleation. The nucleation mechanism accounts for the observed phenomenology and reveals its microscopic origins. We demonstrated that the large lattice distortions that accompany the formation of the diamond nuclei inhibit the phase transition at low pressure and direct it towards the hexagonal diamond phase at higher pressure. The nucleation mechanism proposed in this work is an important step towards a better understanding of structural transformations in a wide range of complex systems such as amorphous carbon and carbon nanomaterials.

cond-mat.mtrl-sci↗

arXiv2010

Ab initio quality study of the graphite-diamond phase coexistence

An interatomic potential for the diamond and graphite phases of carbon has been created using a neural-network (NN) representation of the ab initio potential energy surface. The NN potential combines the accuracy of a first-principle description of both phases with the efficiency of empirical force fields and allows one to perform, for the first time, a molecular dynamics study, of ab initio quality, of the thermodynamics of graphite-diamond coexistence. Good agreement between the experimental and calculated coexistence curves is achieved if nuclear quantum effects are included in the simulation.

cond-mat.mtrl-sci↗

arXiv2010

Ab initio quality neural-network potential for sodium

An interatomic potential for high-pressure high-temperature (HPHT) crystalline and liquid phases of sodium is created using a neural-network (NN) representation of the ab initio potential energy surface. It is demonstrated that the NN potential provides an ab initio quality description of multiple properties of liquid sodium and bcc, fcc, cI16 crystal phases in the P-T region up to 120 GPa and 1200 K. The unique combination of computational efficiency of the NN potential and its ability to reproduce quantitatively experimental properties of sodium in the wide P-T range enables molecular dynamics simulations of physicochemical processes in HPHT sodium of unprecedented quality.

cond-mat.mtrl-sci↗