DFTB+ is a versatile community developed open source software package offering fast and efficient methods for carrying out atomistic quantum mechanical simulations. By implementing various methods approximating density functional theory (DFT), such as the density functional based tight binding (DFTB) and the extended tight binding method, it enables simulations of large systems and long timescales with reasonable accuracy while being considerably faster for typical simulations than the respective ab initio methods. Based on the DFTB framework, it additionally offers approximated versions of various DFT extensions including hybrid functionals, time dependent formalism for treating excited systems, electron transport using non-equilibrium Green’s functions, and many more. DFTB+ can be used as a user-friendly standalone application in addition to being embedded into other software packages as a library or acting as a calculation-server accessed by socket communication. We give an overview of the recently developed capabilities of the DFTB+ code, demonstrating with a few use case examples, discuss the strengths and weaknesses of the various features, and also discuss on-going developments and possible future perspectives.

Currently, there is no systematic way to describe a quantum process with memory solely in terms of experimentally accessible quantities. However, recent technological advances mean we have control over systems at scales where memory effects are non-negligible. The lack of such an operational description has hindered advances in understanding physical, chemical and biological processes, where often unjustified theoretical assumptions are made to render a dynamical description tractable. This has led to theories plagued with unphysical results and no consensus on what a quantum Markov (memoryless) process is. Here, we develop a universal framework to characterise arbitrary non-Markovian quantum processes. We show how a multi-time non-Markovian process can be reconstructed experimentally, and that it has a natural representation as a many body quantum state, where temporal correlations are mapped to spatial ones. Moreover, this state is expected to have an efficient matrix product operator form in many cases. Our framework constitutes a systematic tool for the effective description of memory-bearing open-system evolutions.

We derive a necessary and sufficient condition for a quantum process to be Markovian which coincides with the classical one in the relevant limit. Our condition unifies all previously known definitions for quantum Markov processes by accounting for all potentially detectable memory effects. We then derive a family of measures of non-Markovianity with clear operational interpretations, such as the size of the memory required to simulate a process, or the experimental falsifiability of a Markovian hypothesis.In classical probability theory, a stochastic process is the collection of joint probability distributions of a system's state (described by random variable X) at different times, {P (X k , t k ; X k−1 , t k−1 ; . . . ; X 1 , t 1 ; X 0 , t 0 ) ∀k ∈ N}; to be a valid process, these distributions must additionally satisfy the Kolmogorov consistency conditions [1]. A Markov process is one where the state X k of the system at any time t k only depends conditionally on the state of the system at the previous time step, and not on the remaining history. That is, the conditional probability distributions satisfy P (X k , t k |X k−1 , t k−1 ;. . .; X 0 , t 0 ) = P (X k , t k |X k−1 , t k−1 ) (1) for all k. This simple looking condition has profound implications, leading to a massively simplified description of the stochastic process. The study of such processes forms an entire branch of mathematics, and the evolution of physical systems is frequently approximated to be Markov (when it is not exactly so). This is in part due to the fact that the properties of Markov processes make them easier to manipulate analytically and computationally [2].Implicit in this description of a classical process is the assumption that the value of X j at a given time can be observed without affecting the subsequent evolution. This assumption cannot be valid for quantum processes. In quantum theory, a measurement must be performed to infer the state of system. And the measurement process, in general, must disturb that state. Therefore, unlike its classical counterpart, a generic quantum stochastic process cannot be described without interfering with it [3]. These complications make it challenging to define the process independently of the control operations of the experimenter. From a technical perspective, a serious consequence of this is that joint probability distributions of quantum observables at different times do not satisfy the Kolmogorov conditions [1], and do not constitute stochastic processes in the classical sense.Nevertheless, temporal correlations between observables do play an important role in the dynamics of many open quantum systems, e.g. in the emission spectra of quantum dots [4] and in the vibrational motion of interacting molecular fluids [5]. Quantifying memory effects, and clearly defining * felix.pollock@monash.edu † kavan.modi@monash.edu the boundary between Markovian and non-Markovian quantum processes, represents an important challenge in describing such systems. Attempts at solving this problem tend to take a...

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Great enthusiasm in single-atom catalysts (SACs) for the N2 reduction reaction (NRR) has been aroused by the discovery of Metal (M)−Nx as a promising catalytic center. However, the performance of available SACs, including poor activity and selectivity, is far away from the industrial requirement because of the inappropriate adsorption behaviors of the catalytic centers. Through the first-principles high-throughput screening, we find that the rational construction of Fe−Fe dual-atom centered site distributed on graphite carbon nitride (Fe2/g-CN)compromises the ability to adsorb N2H and NH2, achieving the best NRR performance among 23 different transition metal (TM) centers. Our results show that Fe2/g-CN can achieve a Faradic efficiency of 100% for NH3 production. Impressively, the limiting-potential of Fe2/g-CN is estimated as low as −0.13 V, which is hitherto the lowest value among the reported theoretical results. Multiple-level descriptors (excess electrons on the adsorbed N2 and integrated-crystal orbital Hamilton population) shed light on the origin of NRR activity from the view of energy, electronic structure, and basic characteristics. As a ubiquitous issue during electrocatalytic NRR, ammonia contamination originating from the substrate decomposition can be surmounted. Our predictions offer a new platform for electrocatalytic synthesis of NH3, contributing to further elucidate the structure−performance correlations.

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We present a consistent linear response formulation of the density functional based tight-binding method for long-range corrected exchange-correlation functionals (LC-DFTB). Besides a detailed account of derivation and implementation of the method, we also test the new scheme on a variety of systems considered to be problematic for conventional local/semilocal time-dependent density functional theory (TD-DFT). To this class belong the optical properties of polyacenes and nucleobases, as well as charge transfer excited states in molecular dimers. We find that the approximate LC-DFTB method exhibits the same general trends and similar accuracy as range-separated DFT methods at significantly reduced computational cost. The scheme should be especially useful in the determination of the electronic excited states of very large molecules, for which conventional TD-DFT is supposed to fail due to a multitude of artificial low energy states.

Density functional theory (DFT) calculations based on the self-consistent-charge tight-binding approximation have been performed to study the influence of the protein pocket on the 3-dimensional structure of the 11-cis-retinal Schiff base (SB) chromophore. Starting with an effectively planar chromophore embedded in a protein pocket consisting of the 27 next-nearest amino acids, the relaxed chromophore geometry resulting from energy optimization and molecular dynamics (MD) simulations has yielded novel insights with respect to the following questions: (i) The conformation of the beta-ionone ring. The protein pocket tolerates both conformations, 6-s-cis and 6-s-trans, with a total energy difference of 0.7 kcal/mol in favor of the former. Of the two possible 6-s-cis conformations, the one with a negative twist angle (optimized value: -35 degrees ) is strongly favored, by 3.6 kcal/mol, relative to the one in which the dihedral is positive. (ii) Out-of-plane twist of the chromophore. The environment induces a nonplanar helical deformation of the chromophore, with the distortions concentrated in the central region of the chromophore, from C10 to C13. The dihedral angle between the planes formed by the bonds from C7 to C10 and from C13 to C15 is 42 degrees. (iii) The absolute configuration of the chromophore. The dihedral angle about the C12-C13 bond is +170 degrees from planar s-cis, which imparts a positive helicity on the chromophore, in agreement with earlier considerations based on theoretical and spectroscopic evidence.