High-Dimensional Neural Network Potentials for Accurate Prediction of Equation of State: A Case Study of Methane.

Machine learning-based interatomic potentials, such as those provided by neural networks, are increasingly important in molecular dynamics simulations. In the present work, we consider the applicability and robustness of machine learning molecular dynamics to predict the equation of state properties of methane by using high-dimensional neural network potentials (HDNNPs). We investigate two different strategies for generating training data: one strategy based upon bulk representations using periodic cells and another strategy based upon clusters of molecules. We assess the accuracy of the trained potentials by predicting the equilibrium mass density for a wide range of thermodynamic conditions to characterize the liquid phase, supercritical fluid, and gas phase, as well as the liquid-vapor coexistence curve. Our results show an excellent agreement with reference phase diagrams, with an average error below ∼2% for all studied phases. Moreover, we confirm the applicability of models trained on cluster data sets for producing accurate and reliable results.

An anti-unwinding finite time fault tolerant sliding mode control of a satellite based on accurate estimation of inertia moments.

Design of an adaptive anti-unwinding finite time sliding mode algorithm for attitude control system of a three-axis satellite is approached in this paper. In the designed controller, some parameters updating laws are proposed to simultaneously obtain the accurate values of inertia moments, the upper values of reaction wheels' failures and disturbances. Accordingly, the combined finite time stability of the control system and convergence of the satellite parameters has been demonstrated. Also, a new sliding surface is suggested to avoid the unwinding problem which is common in sliding mode techniques. It is proved by the Lyaponov methodology that the satellite quaternions and angular velocities always approach to the nearest equilibrium point and so it prevents the excessive satellite slews. Furthermore, the proposed algorithm has been modified in such a way that the singularity problem does not occur, especially when the system states reach around the equilibrium point. Therefore, the finite time convergence of system states and parameters estimation errors are guaranteed subjected to disturbances and actuator failures without occurring singularity and unwinding problems. Besides rigorous theoretical proofs by the Lyapunov theory, different simulations are exhibited to demonstrate the merits of the developed control schemes.

Mechanism of Photoinduced Dihydroazulene Ring-Opening Reaction.

The photoinduced ring-opening reaction is a key process in the functioning of dihydroazulene/vinylheptafulvene (DHA/VHF) photoswitches. Over the years, the mechanism of this reaction has been extensively debated. Herein, by means of nonadiabatic trajectory dynamics simulations and quantum chemistry calculations, we present the first detailed and comprehensive investigation on the mechanism of the photoinduced ring-opening reaction of DHA. The results show the crucial role of the excited-state ring planarization process for the bond breaking. Our dynamics simulations show that the DHA ring opening is an ultrafast reaction that does not follow exponential kinetics but exhibits ballistic dynamics. Upon photoexcitation, the planarization occurs within 300-500 fs. This leads to the ring-opening reaction and concurrent decay of the molecule to the ground state within 100 fs through an S1 → S0 internal conversion process toward forming the VHF isomer. These results are consistent with previous ultrafast time-resolved experiments and lead to a thorough understanding of the DHA/VHF photoconversion.

Excited-state solvation structure of transition metal complexes from molecular dynamics simulations and assessment of partial atomic charge methods.

In this work, we investigate the excited-state solute and solvation structure of [Ru(bpy)3]2+, [Fe(bpy)3]2+, [Fe(bmip)2]2+ and [Cu(phen)2]+ (bpy = 2,2'-bipyridine; bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine; phen = 1,10-phenanthroline) transition metal complexes (TMCs) in terms of solute-solvent radial distribution functions (RDFs) and evaluate the performance of some of the most popular partial atomic charge (PAC) methods for obtaining these RDFs by molecular dynamics (MD) simulations. To this end, we compare classical MD of a frozen solute in water and acetonitrile (ACN) with quantum mechanics/molecular mechanics Born-Oppenheimer molecular dynamics (QM/MM BOMD) simulations. The calculated RDFs show that the choice of a suitable PAC method is dependent on the coordination number of the metal, denticity of the ligands, and type of solvent. It is found that this selection is less sensitive for water than ACN. Furthermore, a careful choice of the PAC method should be considered for TMCs that exhibit a free direct coordination site, such as [Cu(phen)2]+. The results of this work show that fast classical MD simulations with ChelpG/RESP or CM5 PACs can produce RDFs close to those obtained by QM/MM MD and thus, provide reliable solvation structures of TMCs to be used, e.g. in the analysis of scattering data.

Mismanagement of a hypochondriacal patient.

Hypochondriasis is a persistent preoccupation that despite appropriate medical evaluations and assurance of patient's physical health, the patient insists on having a serious disease. The case which is discussed in this article is a 39-years-old woman that hospitalized for half of her life and no one can perceive her disorder according to her assertions. The mentioned case is a "difficult patient" with fear of oxygen shortage and being choked (Pnigophobia) which leads to continuous tendency to wear oxygen device even during sleep. There is no benefit in exaggerating her condition for herself so there is no fictitious disorder considered. During the therapy she has been assured that she does not have a serious disease and she has learnt to decrease oxygen intake and breath with his mouth. The point that makes this study different from the others is that most of hypochondriacal patients have a fear of getting HIV, cancers, hepatitis and MS but our patient has phobia of pulmonary embolism.

Theoretical studies on the tautomerism of tetrazole selenone.

The tautomerism of all possible forms of tetrazole selenone (A-G), induced by proton transfer, was studied, theoretically, in different environments including gas phase, continuum solvent and microsolvated environment with one or two explicit water or ammonia molecules. The calculations were performed using two different levels of theory including mPW2PLYP and DFT-B3LYP. The 6-311++G(d,p) basis set was used for C, H, O and N and the standard relativistic effective core pseudo potential LANL2DZ basis set was used for Se atom. It was found that the tetrazole selenone, in the form of A, is the most stable isomer in all of the environments considered in this work. The kinetics of proton transfer reaction was studied in both gas and solvent environments and it was concluded that the activation energy of the reaction increases with going from the gas phase to polar solvents. Moreover, the proton transfer reaction assisted by one or two water or ammonia molecules was investigated and it was found that the activation energy significantly reduces.

Acidity constants of some sulfur oxoacids in aqueous solution using CCSD and MP2 methods.

The calculations of the acidity constants (pKa) of a series of sulfur oxoacids including H2SOn (n = 1-5) and H2S2On (n = 1, 3, 4, 6 and 7) are presented for the first time. The calculations were performed using two expensive correlated levels of theory including MP2/6-311++G(3df,3pd) and CCSD/6-311++G(d,p) in both gas and aqueous phases. The new continuum solvation model, SMD, based on the quantum mechanical charge density of a solute molecule interacting with a continuum description of the solvent, used to account the solvent effects. The calculated pKas were corrected using the different correlation equations (Zimmermann and Tossell, J. Phys. Chem. A, 2009, 113, 5105-5111) to improve the accuracy of results. Also, the calculated results showed the effect of the intramolecular hydrogen bonding on the acidity strength.