32 research outputs found
i-PI 2.0: A Universal Force Engine for Advanced Molecular Simulations
Progress in the atomic-scale modeling of matter over the past decade has been tremendous. This progress has been brought about by improvements in methods for evaluating interatomic forces that work by either solving the electronic structure problem explicitly, or by computing accurate approximations of the solution and by the development of techniques that use the Born–Oppenheimer (BO) forces to move the atoms on the BO potential energy surface. As a consequence of these developments it is now possible to identify stable or metastable states, to sample configurations consistent with the appropriate thermodynamic ensemble, and to estimate the kinetics of reactions and phase transitions. All too often, however, progress is slowed down by the bottleneck associated with implementing new optimization algorithms and/or sampling techniques into the many existing electronic-structure and empirical-potential codes. To address this problem, we are thus releasing a new version of the i-PI software. This piece of software is an easily extensible framework for implementing advanced atomistic simulation techniques using interatomic potentials and forces calculated by an external driver code. While the original version of the code (Ceriotti et al., 2014) was developed with a focus on path integral molecular dynamics techniques, this second release of i-PI not only includes several new advanced path integral methods, but also offers other classes of algorithms. In other words, i-PI is moving towards becoming a universal force engine that is both modular and tightly coupled to the driver codes that evaluate the potential energy surface and its derivatives
Fully Consistent Density Functional Theory Determination of the Insulator-Metal Transition Boundary in Warm Dense Hydrogen
Using conceptually and procedurally consistent density functional theory
(DFT) calculations with an advanced meta-GGA exchange-correlation functional in
ab initio molecular dynamics simulations, we determine the insulator-metal
transition (IMT) of warm dense fluid hydrogen over the pressure range 50 to 300
GPa. Inclusion of nuclear quantum effects via path-integral molecular dynamics
(PIMD) sharpens the metallic transition and lowers the transition temperature
relative to results from Born-Oppenheimer (BO) MD. BOMD itself gives improved
agreement with experimental results compared to previous DFT predictions.
Examination of the ionic pair correlation function in the context of the abrupt
conductivity increase at the transition confirms a metallic transition due to
the dissociation of molecular hydrogen that coincides with an abrupt band gap
closure. Direct comparison of the PIMD and BOMD results clearly demonstrates an
isotope effect on the IMT. Distinct from stochastic simulations, these results
do not depend upon any ad hoc combination of ground-state and finite-T
methodologies
The FHI-aims Code: All-electron, ab initio materials simulations towards the exascale
FHI-aims is a quantum mechanics software package based on numeric atom-centered orbitals (NAOs) with broad capabilities for all-electron electronic-structure calculations and ab initio molecular dynamics. It also connects to workflows for multi-scale and artificial intelligence modeling
Ab initio thermodynamics of liquid and solid water
Thermodynamic properties of liquid water as well as hexagonal (Ih) and cubic
(Ic) ice are predicted based on density functional theory at the
hybrid-functional level, rigorously taking into account quantum nuclear motion,
anharmonic fluctuations and proton disorder. This is made possible by combining
advanced free energy methods and state-of-the-art machine learning techniques.
The ab initio description leads to structural properties in excellent agreement
with experiments, and reliable estimates of the melting points of light and
heavy water. We observe that nuclear quantum effects contribute a crucial 0.2
meV/HO to the stability of ice Ih, making it more stable than ice Ic. Our
computational approach is general and transferable, providing a comprehensive
framework for quantitative predictions of ab initio thermodynamic properties
using machine learning potentials as an intermediate step
The importance of nuclear quantum effects for NMR crystallography
The resolving power of solid-state nuclear magnetic resonance (NMR)
crystallography depends heavily on the accuracy of the computational prediction
of NMR chemical shieldings of candidate structures, which are usually taken to
be local minima in the potential energy surface. To test the limits of this
approximation, we perform a systematic study of the role of finite-temperature
and quantum nuclear fluctuations on H, C, and N chemical
shieldings in molecular crystals -- considering the paradigmatic examples of
the different polymorphs of benzene, glycine, and succinic acid. We find the
effect of quantum fluctuations to be comparable in size to the typical errors
of predictions of chemical shieldings for static nuclei with respect to
experimental measurements, and to improve the match between experiments and
theoretical predictions, translating to more reliable assignment of the NMR
spectra to the correct candidate structure. Thanks to the use of integrated
machine-learning models trained on both first-principles configurational
energies and chemical shieldings, the accurate sampling of thermal and quantum
fluctuations of the structures can be achieved at an affordable cost, setting a
new standard for the calculations that underlie solid-state structural
determination by NMR
Ultrafast charge transfer and vibronic coupling in a laser-excited hybrid inorganic/organic interface
Hybrid interfaces formed by inorganic semiconductors and organic molecules are intriguing materials for opto-electronics. Interfacial charge transfer is primarily responsible for their peculiar electronic structure and optical response. Hence, it is essential to gain insight into this fundamental process also beyond the static picture. Ab initio methods based on real-time time-dependent density-functional theory coupled to the Ehrenfest molecular dynamics scheme are ideally suited for this problem. We investigate a laser-excited hybrid inorganic/organic interface formed by the electron acceptor molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ) physisorbed on a hydrogenated silicon cluster, and we discuss the fundamental mechanisms of charge transfer in the ultrashort time window following the impulsive excitation. The considered interface is p-doped and exhibits charge transfer in the ground state. When it is excited by a resonant laser pulse, the charge transfer across the interface is additionally increased, but contrary to previous observations in all-organic donor/acceptor complexes, it is not further promoted by vibronic coupling. In the considered time window of 100 fs, the molecular vibrations are coupled to the electron dynamics and enhance intramolecular charge transfer. Our results highlight the complexity of the physics involved and demonstrate the ability of the adopted formalism to achieve a comprehensive understanding of ultrafast charge transfer in hybrid materials
Predicting the Electronic Density Response of Condensed-Phase Systems to Electric Field Perturbations
We present a local and transferable machine learning approach capable of
predicting the real-space density response of both molecules and periodic
systems to external homogeneous electric fields. The new method, SALTER, builds
on the Symmetry-Adapted Gaussian Process Regression SALTED framework. SALTER
requires only a small, but necessary, modification to the descriptors used to
represent the atomic environments. We present the performance of the method on
isolated water molecules, bulk water and a naphthalene crystal. Root mean
square errors of the predicted density response lie at or below 10% with barely
more than 100 training structures. Derived quantities, such as polarizability
tensors and even Raman spectra further derived from these tensors show a good
agreement with those calculated directly from quantum mechanical methods.
Therefore, SALTER shows excellent performance when predicting derived
quantities, while retaining all of the information contained in the full
electronic response. This method is thus capable of learning vector fields in a
chemical context and serves as a landmark for further developments.Comment: Main text: 7 pages, 5 figures. SI: 5 pages, 4 figure
Improving Molecular Force Fields Across Configurational Space by Combining Supervised and Unsupervised Machine Learning
The training set of atomic configurations is key to the performance of any
Machine Learning Force Field (MLFF) and, as such, the training set selection
determines the applicability of the MLFF model for predictive molecular
simulations. However, most atomistic reference datasets are inhomogeneously
distributed across configurational space (CS), thus choosing the training set
randomly or according to the probability distribution of the data leads to
models whose accuracy is mainly defined by the most common close-to-equilibrium
configurations in the reference data. In this work, we combine unsupervised and
supervised ML methods to bypass the inherent bias of the data for common
configurations, effectively widening the applicability range of MLFF to the
fullest capabilities of the dataset. To achieve this goal, we first cluster the
CS into subregions similar in terms of geometry and energetics. We iteratively
test a given MLFF performance on each subregion and fill the training set of
the model with the representatives of the most inaccurate parts of the CS. The
proposed approach has been applied to a set of small organic molecules and
alanine tetrapeptide, demonstrating an up to two-fold decrease in the root mean
squared errors for force predictions of these molecules. This result holds for
both kernel-based methods (sGDML and GAP/SOAP models) and deep neural networks
(SchNet model). For the latter, the developed approach simultaneously improves
both energy and forces, bypassing the compromise to be made when employing
mixed energy/force loss functions