spring
| Format: | spring N C(2) k2 qcore qshell … |
| Description: | This keyword turns on core-shell model due to Dick and Overhauser to account for atomic polarizability. In this model electron cloud is represented as a massless shell that is connected to a core by the harmonic potential of the form: N defines a number of atom types for which shell model will be used. String constant C(2) specifies the atom type (2 symbols maximum) that must be defined by the species keyword. The units for spring constant k2 is eV/angstrom2, core and shell charges qcore and qshell are in the units of electron charge. While core and shell each has their own charges, core does not interact electrostatically with its own shell, but does so with all other cores and shells in the system. Consequently total ionic charge specified by species keyword becomes irrelevant if given atom type is described by the shell model. Note, that while second derivatives are rigorously avalable analytically for this approach, third derivatives can be calculated only in an approximate way, which is not very accurate. It is therefore recommended to set numerical_3der, so third derivatives would be evaluated numerically. |
| Examples: | spring 2 O 80.21 1.14 -3.06 U 210.02 -4.1 7.94 |
spring4
| Format: | spring N C(2) k4 … |
| Description: | This keyword adds additional, 4th power term to core-shell interaction. Such additional term usually is important in description of ferroelectric materials as in this example. Potential if of the form: N defines a number of atom types for which this term will be used. String constant C(2) specifies the atom type (2 symbols maximum) that must be defined by the species keyword. The units for spring constant k4 is eV/angstrom4. |
| Examples: | spring4 1 Ba 400. |
frac_shell
| Format: | frac_shell C(2) s R1 R2 R3 … |
| Description: | This is to provide the initial shell positions for the calculations in terms of the lattice vectors. “s” is the actual letter, as see in examples, and C(2) is the name of the atom, R1,R2,R3 are the x,y and z fractional coordinates. If this keyword is omitted, but shell model is considered, then initial positions of the shells are right on top of the cores. |
| Examples: | frac_shell U s 0.00 0.00 0.00 U s 0.00 0.50 0.50 U s 0.50 0.00 0.50 U s 0.50 0.50 0.00 O s 0.25 0.25 0.25 O s 0.75 0.25 0.25 O s 0.75 0.75 0.25 O s 0.25 0.75 0.25 O s 0.25 0.25 0.75 O s 0.75 0.25 0.75 O s 0.75 0.75 0.75 O s 0.25 0.75 0.75 |
cart_shell
| Format: | cart_shell C(2) s R1 R2 R3 … |
| Description: | This is to provide the initial shell positions for the calculations in cartesian coordinates. “s” is the actual letter, as see in examples, and C(2) is the name of the atom, R1,R2,R3 are the x,y and z coordinates in angstroms. If this keyword is omitted, but shell model is considered, then initial positions of the shells are right on top of the cores. |
| Examples: | Cart_shell U s 0.00 0.00 0.00 U s 0.00 2.74 2.74 U s 2.74 0.00 2.74 U s 2.74 2.74 0.00 O s 1.37 1.37 1.37 O s 4.11 1.37 1.37 O s 4.11 4.11 1.37 O s 1.37 4.11 1.37 O s 1.37 1.37 4.11 O s 4.11 1.37 4.11 O s 4.11 4.11 4.11 O s 1.37 4.11 4.11 |