pair_style smtbq command
Syntax
pair_style smtbq
Examples
pair_style smtbq
pair_coeff * * ffield.smtbq.Al2O3 O Al
Description
This pair style computes a variable charge SMTB-Q (Second-Moment tight-Binding QEq) potential as described in SMTB-Q_1 and SMTB-Q_2. This potential was first proposed in SMTB-Q_0. Briefly, the energy of metallic-oxygen systems is given by three contributions:
where
The parameters appearing in the upper expressions are set in the
ffield.SMTBQ.Syst file where Syst corresponds to the selected system
(e.g. field.SMTBQ.Al2O3). Examples for
pair_coeff * * PathToLammps/potentials/ffield.smtbq.TiO2 O Ti
The electrostatic part of the energy consists of two components
self-energy of atom i in the form of a second order charge dependent
polynomial and a long-range Coulombic electrostatic interaction. The
latter uses the wolf summation method described in Wolf,
spherically truncated at a longer cutoff,
Interaction between oxygen,
The short-range interaction between metal-oxygen,
where
In the formalism used here,
Thus parameter
The potential offers the possibility to consider the polarizability of
the electron clouds of oxygen by changing the slater radius of the
charge density around the oxygen atoms through the parameters rBB, rB and
rS in the ffield.SMTBQ.Syst. This change in radius is performed
according to the method developed by E. Maras
SMTB-Q_2. This method needs to determine the number of
nearest neighbors around the oxygen. This calculation is based on
first (
The SMTB-Q potential is a variable charge potential. The equilibrium charge on each atom is calculated by the electronegativity equalization (QEq) method. See Rick for further detail. One can adjust the frequency, the maximum number of iterative loop and the convergence of the equilibrium charge calculation. To obtain the energy conservation in NVE thermodynamic ensemble, we recommend to use a convergence parameter in the interval 10e-5 - 10e-6 eV.
The ffield.SMTBQ.Syst files are provided for few systems. They consist of nine parts and the lines beginning with ‘#’ are comments (note that the number of comment lines matter). The first sections are on the potential parameters and others are on the simulation options and might be modified. Keywords are character type and must be enclosed in quotation marks (‘’).
Number of different element in the oxide:
N_elem= 2 or 3
Divider line
Atomic parameters
For the anion (oxygen)
Name of element (char) and stoichiometry in oxide
Formal charge and mass of element
Principal quantum number of outer orbital n), electronegativity (
) and hardness ( )Ionic radius parameters : max coordination number (coordBB = 6 by default), bulk coordination number (coordB), surface coordination number (coordS) and rBB, rB and rS the slater radius for each coordination number. (note : If you don’t want to change the slater radius, use three identical radius values)
Number of orbital shared by the element in the oxide (
)Divider line
For each cations (metal):
Name of element (char) and stoichiometry in oxide
Formal charge and mass of element
Number of electron in outer orbital (ne), electronegativity (
), hardness ( ) and the slater radius for the cation.Number of orbitals shared by the elements in the oxide (
)Divider line
Potential parameters:
Keyword for element1, element2 and interaction potential (‘second_moment’ or ‘buck’ or ‘buckPlusAttr’) between element 1 and 2. If the potential is ‘second_moment’, specify ‘oxide’ or ‘metal’ for metal-oxygen or metal-metal interactions respectively.
Potential parameter:
If type of potential is ‘second_moment’ : A (eV), p,
(eV) and q, , andIf type of potential is ‘buck’ : C (eV) and
If type of potential is ‘buckPlusAttr’ : C (eV) and
D (eV), B , and
Divider line
Tables parameters:
Cutoff radius for the Coulomb interaction (
)Starting radius (
) and increments ( ) for creating the potential table.Divider line
Rick model parameter:
Nevery : parameter to set the frequency of the charge resolution. The charges are evaluated each Nevery time steps.
Max number of iterative loop (loopmax) and convergence criterion (prec) in eV of the charge resolution
Divider line
Coordination parameter:
First (
) and second ( ) neighbor distances in angstromsDivider line
Charge initialization mode:
Keyword (QInitMode) and initial oxygen charge (
). If keyword = ‘true’, all oxygen charges are initially set equal to . The charges on the cations are initially set in order to respect the neutrality of the box. If keyword = ‘false’, all atom charges are initially set equal to 0 if you use the create_atoms command or the charge specified in the file structure using read_data command.Divider line
Mode for the electronegativity equalization (Qeq)
Keyword (mode) followed by:
QEqAll (one QEq group) | no parameters
QEqAllParallel (several QEq groups) | no parameters
Surface | zlim (QEq only for z>zlim)
Parameter if necessary
Divider line
Verbose
If you want the code to work in verbose mode or not : ‘true’ or ‘false’
If you want to print or not in the file ‘Energy_component.txt’ the three main contributions to the energy of the system according to the description presented above : ‘true’ or ‘false’ and
. This option writes to the file every time steps. If the value is ‘false’ then . The file takes into account the possibility to have several QEq groups g then it writes: time step, number of atoms in group g, electrostatic part of energy, , the interaction between oxygen, , and short range metal-oxygen interaction, .If you want to print to the file ‘Electroneg_component.txt’ the electronegativity component (
) or not: ‘true’ or ‘false’ and . This option writes to the file every time steps. If the value is ‘false’ then . The file consist of atom number i, atom type (1 for oxygen and # higher than 1 for metal), atom position: x, y and z, atomic charge of atom i, electrostatic part of atom i electronegativity, covalent part of atom i electronegativity, the hopping integral of atom i and box electronegativity.
Note
This last option slows down the calculation dramatically. Use only with a single processor simulation.
Mixing, shift, table, tail correction, restart, rRESPA info
This pair style does not support the pair_modify mix, shift, table, and tail options.
This pair style does not write its information to binary restart files, since it is stored in potential files. Thus, you needs to re-specify the pair_style and pair_coeff commands in an input script that reads a restart file.
This pair style can only be used via the pair keyword of the run_style respa command. It does not support the inner, middle, outer keywords.
Restrictions
This pair style is part of the SMTBQ package and is only enabled if LAMMPS is built with that package. See the Build package page for more info.
This potential requires using atom type 1 for oxygen and atom type higher than 1 for metal atoms.
This pair style requires the newton setting to be “on” for pair interactions.
The SMTB-Q potential files provided with LAMMPS (see the potentials directory) are parameterized for metal units.
Citing this work
Please cite related publication: N. Salles, O. Politano, E. Amzallag and R. Tetot, Comput. Mater. Sci. 111 (2016) 181-189
(SMTB-Q_0) A. Hallil, E. Amzallag, S. Landron, R. Tetot, Surface Science 605 738-745 (2011); R. Tetot, A. Hallil, J. Creuze and I. Braems, EPL, 83 40001 (2008)
(SMTB-Q_1) N. Salles, O. Politano, E. Amzallag, R. Tetot, Comput. Mater. Sci. 111 (2016) 181-189
(SMTB-Q_2) E. Maras, N. Salles, R. Tetot, T. Ala-Nissila, H. Jonsson, J. Phys. Chem. C 2015, 119, 10391-10399
(SMTB-Q_3) R. Tetot, N. Salles, S. Landron, E. Amzallag, Surface Science 616, 19-8722 28 (2013)
(Wolf) D. Wolf, P. Keblinski, S. R. Phillpot, J. Eggebrecht, J Chem Phys, 110, 8254 (1999).
(Rick) S. W. Rick, S. J. Stuart, B. J. Berne, J Chem Phys 101, 6141 (1994).