This repository contains program “Deductive Molecular Mechanics of Carbon Allotropes” designed to calculate energy and mechanical propetries of carbon allotropes within the group function approach. Present version of the program supports only allotropes, allowing pure localized SLG treatment i.e. not containing delocalized pi-system.
To run the program one needs to have two input files called ‘dmmin’ and ‘POSCAR’. Second one is just a standard POSCAR file used in the VASP package (POSCAR file for diamond is available in the directory ‘Examples’).
File ‘dmmin’ contains several settings (5 in present version), which needs to be specified by User. Each setting has its own fixed keyword and may have several possible values.
First setting is a parameterization scheme (way to parametrize semi-empirical Hamiltonian) which is going to be used. Keyword used for this setting is ‘PARAMETERIZATION:’ (all keywords may be written by lowercase also). Present version of the program supports two schemes: CNDO and MNDO, thus possible values of present setting are ‘CNDO’ and ‘MNDO’ respectively.
Second setting is approximation used to evaluate SLG reduced density matrices (RDMs). Corresponding keyword is ‘SLG_DENSITY:’. This setting has two possible values: ‘mean’ and ‘corr’. First one corresponds to mean field approximation and second one takes correlation corrections into account.
Third setting is the basis set which is going to be used. Key word: ‘BASIS_SET:’. Present version of the program supports three types of basis sets: i) basis of single Slater type orbitals; ii) two parametric hydrogen-like basis set (H-2 set); iii) three parametric hydrogen-like basis set (H-3 set). Respective values of this setting are ‘STO’, ‘H-2’ and ‘H-3’.
Orbital exponents have default values for each basis set (for STO they are exponents corresponding to parameterization scheme used, for H-2 and H-3 sets they are values obtained by minimizing energy of free atom). However, values of orbital exponents may be varied by User. To do this key word ‘ORBITAL_EXPONENTS:’ may be used. If default values of orbital exponents are plaining to be used, then this setting should have value ‘Default’. Otherwise User should specify orbital exponents after this key word. It should be noted that STO basis requires one exponent, H-2 requires two exponents and H-3 requires three exponents. For H-2 basis exponents should be written in the order: 2s, 2p. For H-3 basis in the order: 1s, 2s, 2p.
Resonance parameters of carbon may either have standard values (depending on parameterization) or may be specified by user. To enter resonance parameters keyword ‘RESONANCE_PARAMETERS:’ is used. If values of resonance parameters are going to be standard, then ‘Default’ value should be assigned to this keyword. Otherwise User should specify resonance parameters in the following order: ss, sp, pp, PiPi.
Another setting is a number of layers used to evaluate nonbond interactions. Key word: ‘NUMBER_OF_LAYERS:’. This parameter shows, how many layers of unit cells are going to be taken into account in nonbond energy calculations. Its value corresponds to the maximum number of translations on each unit vector. Default value of this setting is 7.
Several examples of ‘dmmin’ file are collected in the directory ‘Examples’. Please note, that described keywords may be presented in any arbitrary order in ‘dmmin’ file.
As the result program creates two output files. First one called ‘geometry.xyz’ contains Cartesian coordinates of atoms located in the super cell build from 27 unit cells (one main unit cell and 26 neighbouring). This file may be used to visualize geometries of allotropes.
Second output file called ‘dmmout’ contains the following information:
Total energy of the system and values of several energy contributions: one center, bonding energy and nonbond energy.
Optimized values of HOs expansion coefficients for each atom in the unit cell (vector parts are given in the laboratory coordinate frame).
This output file needs to be supplemented with force constant matrices and bulk modulus (may be some other information is also required - we need to discuss it).