I tried to use the reaxff-lg with GULP, but my structures collapsed. To start, I inspected the library provided with 6.1.2 version, reaxff_lg.lib, where the -lg part is included as
C core C core 0.0 3.8510 0.0 10.0
C core O core 632.0 3.6755 0.0 10.0
C core N core 650.0 3.7555 0.0 10.0
C core H core 0.0 3.3685 0.0 10.0
O core O core 624.0 3.5000 0.0 10.0
O core N core 880.0 3.5800 0.0 10.0
O core H core 0.0 3.1940 0.0 10.0
N core N core 1239.0 3.6600 0.0 10.0
N core H core 295.0 3.2730 0.0 10.0
cutp 10.0 p7
The GULP manual/help entry for becke_johnson_c6 is:
Format becke_johnson_c6 <intra/inter> <bond/x12/x13/x14/mol/o14/g14> <kcal/kjmol>
atom1 atom2 C6 r0 rmax <2flags>
Units C6 in eVAngs6, r0 in Angs
Use Specifies potential parameters for the Becke-Johnson form of damped C6 term.
The energy is of the form:
E = - C6/(r6 + r0**6)
In other words, in the example above, for N-N interaction we have the C6=1239.0 and r0=3.6600 Angs.
The value of r0 in other sources is significantly lower, i.e. around 1.8 (see for example
dx.doi.org/10.1021/jp201599t | J. Phys. Chem. A 2011, 115, 11016–11022 ) where the values are C6=1239 but R0 is 1.83.
Can you please tell me why the R0 values are so different, and which one is correct for N?