Minima Hopping for Crystal Structure Optimization

The minima hopping method implements a global optimization technique to identify the ground state structure of any chemical system.

Input/Output files

In addition to the standard flame_in.yaml file, further input files are required to start a MHM run. Also, important information will be written into output files other than flame_log.yaml, e.g., earr.dat and global.mon.

poscur.ascii/poscur.vasp: intitial or current configuration of a MHM run either in the ascii or vasp format, in Angstrom. It will be overwritten continuously as the simulation progresses with the current local minimum structure.

ioput: contains parameter values which are read and continuously updated throughout an MHM run. In one line, 3 real numbers must be provided, which correspond to ediff (in Ha/cell), temperature (in K and scaled based on the atomic masses amass), maximum temperature (in K and scaled based on the atomic masses amass).

Suggested initial choice is 1.d-2 5.d2 1.d5 (for a system with around 16 atoms)

earr.dat: contains information about the initial/current state of the MHM run, and is populated with structural data during the simulation. The first two lines must be always provided:

• Line 1: nmin_cur, nmin_max

• Line 2: delta_enthalpy, delta_fingerprint

nmin_cur is the currently known number of local minima, and nmin_max is the maximal number of minima to be found in the next MHM run. nmin_max must always be larger than nmin_cur. Every new MHM run must start with nmin_cur: 0 In a restart run, nmin_cur corresponds to the number of local minima found in the previous run(s).

delta_enthalpy (units of Ha/cell) and delta_fingerprint are the minimal enthalpies and structural fingerprint tolerances, respectively, below which two structures will be considered identical.

Suggested initial choice: 2.d-3 1.d-1 (for a system with around 16 atoms and the Oganov fingerprint method)

During an MHM run, the value nmin_cur will incrementally increase, and addtional nmin_cur lines will be written to earr.dat, each corresponding to a distinct local minimum. This list of local minima is constantly sorted based on the enthalpy. Every line consists of 6 entries:

• Line i + 2: imin, enthalpy, energy, nvisits, spg, spg_tol

where imin is the index of the local minimum, enthalpy and energy are its enthalpy/energy (in Ha/cell), nvisits is the number of times it has been encountered during the run, spg is its space group index (for crystalline systems), and spg_tol is the tolerance used in SPGLIB to determine the space group.

poslowXXXXX.ascii/poslowXXXXX.vasp: The structures corresponding to the nmin_cur local minima are written into separate files, where XXXXX corresponds to the index imin in earr.dat.

minhocao options

nsoften: (integer) Number of softening iterations to eliminate the high-frequency modes from the initial MD velocities.

default: 7

alpha_at: (real) Stepsize for the softening algorithm on the atomic degrees of freedom. Arbitrary units.

default: 5.d-1

alpha_lat: (real) Stepsize for the softening algorithm on the lattice degrees of freedom. Arbitrary units.

default: 5.d-1

auto_soften: (logical) Choice of automatically adjusting the stepsize of the softening iterations based on gradient feedback.

default: True

eref: (real) Reference energy, MHM run will stop as soon a minimum with an energy/enthlapy less than eref is found. In units of Ha/cell.

default: -1.d50