Publications

2024

1. Surface Phase Diagrams from Nested Sampling

   Mingrui Yang , Livia B. Pártay , and Robert B. Wexler

   Physical Chemistry Chemical Physics, May 2024

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   Studies in atomic-scale modeling of surface phase equilibria often focus on temperatures near zero Kelvin due to the challenges in calculating the free energy of surfaces at finite temperatures. The Bayesian-inference-based nested sampling (NS) algorithm allows for modeling phase equilibria at arbitrary temperatures by directly and efficiently calculating the partition function, whose relationship with free energy is well known. This work extends NS to calculate adsorbate phase diagrams, incorporating all relevant configurational contributions to the free energy. We apply NS to the adsorption of Lennard-Jones (LJ) gas particles on low-index and vicinal LJ solid surfaces and construct the canonical partition function from these recorded energies to calculate ensemble averages of thermodynamic properties, such as the constant-volume heat capacity and order parameters that characterize the structure of adsorbate phases. Key results include determining the nature of phase transitions of adsorbed LJ particles on flat and stepped LJ surfaces, which typically feature an enthalpy-driven condensation at higher temperatures and an entropy-driven reordering process at lower temperatures, and the effect of surface geometry on the presence of triple points in the phase diagrams. Overall, we demonstrate the ability and potential of NS for surface modeling.

2022

1. Stochastic Differential Equation Approach to Understanding the Population Control Bias in Full Configuration Interaction Quantum Monte Carlo

   Joachim Brand , Mingrui Yang , and Elke Pahl

   Physical Review B, Jun 2022
2. Polaron-Depleton Transition in the Yrast Excitations of a One-Dimensional Bose Gas with a Mobile Impurity

   Mingrui Yang , Matija Čufar , Elke Pahl , and 1 more author

   Condensed Matter, Mar 2022

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   We present exact numerical data for the lowest-energy momentum eigenstates (yrast states) of a repulsive spin impurity in a one-dimensional Bose gas using full configuration interaction quantum Monte Carlo (FCIQMC). As a stochastic extension of exact diagonalization, it is well suited for the study of yrast states of a lattice-renormalized model for a quantum gas. Yrast states carry valuable information about the dynamic properties of slow-moving mobile impurities immersed in a many-body system. Based on the energies and the first and second-order correlation functions of yrast states, we identify different dynamical regimes and the transitions between them: The polaron regime, where the impurity’s motion is affected by the Bose gas through a renormalized effective mass; a regime of a gray soliton that is weakly correlated with a stationary impurity, and the depleton regime, where the impurity occupies a dark or gray soliton. Extracting the depleton effective mass reveals a super heavy regime where the magnitude of the (negative) depleton mass exceeds the mass of the finite Bose gas.

2021

1. A Theoretical Investigation of 38-Atom CuPd Clusters: The Effect of Potential Parameterisation on Structure and Segregation

   Caitlin A. Casey-Stevens , Mingrui Yang , Geoffrey R. Weal , and 3 more authors

   Physical Chemistry Chemical Physics, Mar 2021

2020

1. Improved Walker Population Control for Full Configuration Interaction Quantum Monte Carlo

   Mingrui Yang , Elke Pahl , and Joachim Brand

   The Journal of Chemical Physics, Nov 2020

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   Full configuration interaction quantum Monte Carlo (FCIQMC) is a stochastic approach for finding the ground state of a quantum manybody Hamiltonian. It is based on the dynamical evolution of a walker population in Hilbert space, which samples the ground state configuration vector over many iterations. Here, we present a modification of the original protocol for walker population control of Booth et al. [J. Chem. Phys. 131, 054106 (2009)] in order to achieve equilibration at a pre-defined average walker number and to avoid walker number overshoots. The dynamics of the walker population is described by a noisy damped harmonic oscillator and controlled by two parameters responsible for damping and forcing, respectively, for which reasonable values are suggested. We further introduce a population growth witness that can be used to detect annihilation plateaus related to overcoming the FCIQMC sign problem. Features of the new population control procedure such as precise walker number control and fast equilibration are demonstrated. The standard error of the shift estimator for the ground state energy as well as the population control bias is found to be unaffected by the population control procedure or its parameters. The improved control of the walker number, and thereby memory consumption, is a desirable feature required for automating FCIQMC calculations and requires minimal modifications to the existing code.

2019

1. Predictable Electronic Tuning By Choice of Azine Substituent in Five Iron(II) Triazoles: Redox Properties and DFT Calculations

   Santiago Rodríguez-Jiménez , Luca Bondì , Mingrui Yang , and 2 more authors

   Chemistry – An Asian Journal, Nov 2019

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   Five new mononuclear iron(II) tris-ligand complexes, and four solvatomorphs, have been made from the azine-substituted 1,2,4-triazole ligands (Lazine): [FeII(Lpyridazine)3](BF4)2 (1), [FeII(Lpyrazine)3](BF4)2 (2), [FeII(Lpyridine)3](BF4)2 (3), [FeII(L2pyrimidine)3](BF4)2 (4), and [FeII(L4pyrimidine)3](BF4)2 (5). Single-crystal XRD and solid-state magnetometry reveal that all of them are low-spin (LS) iron(II), except for solvatomorph 5⋅4 H2O. Evans method NMR studies in CD2Cl2, (CD3)2CO and CD3CN show that all are LS in these solvents, except 5 in CD2Cl2 (consistent with L4pyrimidine imposing the weakest field). Cyclic voltammetry in CH3CN vs. Ag/0.01 m AgNO3 reveals an, at best quasi-reversible, FeIII/II redox process, with Epa increasing from 0.69 to 0.99 V as the azine changes: pyridine< pyridazine<2-pyrimidine<4-pyrimidine< pyrazine. The observed Epa values correlate linearly with the DFT calculated HOMO energies for the LS complexes.

2017

1. A Simple Method of Predicting Spin State in Solution

   Santiago Rodríguez-Jiménez , Mingrui Yang , Ian Stewart , and 2 more authors

   Journal of the American Chemical Society, Dec 2017

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   A simple method, using density functional theory (DFT), of predicting spin-state in advance of synthesis is reported. Specifically, an excellent correlation is observed between the switching temperatures (T1/2) in CDCl3 solution of five spin-crossover (SCO)-active [FeII(Lazine)2(NCBH3)2] complexes and the DFT-calculated (and observed) 15N NMR chemical shift (δNA) of the five different azine-substituted 1,2,4-triazole ligands employed, Lazine = 4-(4-methylphenyl)-3-phenyl-5-(azine)-1,2,4-triazole, where azine = pyridine, pyridazine, 4-pyrimidine, pyrazine, and 2-pyrimidine. To test the generality of this finding, DFT was also employed to readily predict the δNA values for a family of 16 literature ligands, known as bppX,Y [X,Y-substituted 2,6-(pyrazol-1-yl)pyridines], which have produced 16 SCO-active [FeII(bppX,Y)2](Z)2 complexes (Z = BF4 or in one case PF6) in (CD3)2CO solution: again an excellent correlation was found between the computed δNA and the observed T1/2. These correlations represent a key advance in the field, as they allow a simple DFT calculation on a modified ligand to be used to reliably predict, before synthesis of the ligand or complex, the T1/2 that would result from that modification. Achieving such easily predictable tuning of T1/2, and hence of spin-state, is a significant step forward in the field of SCO and also has big implications in many other fields in which spin-state is key, including catalysis, metallo-enzyme modeling studies, and host-guest chemistry.