fix_qeq_reax.cpp in ReaxFF


We use LAMMPS with REAX/C. In “fix_qeq_reax.cpp” , I do not understand the factor conversion #define EV_TO_KCAL_PER_MOL 14.4 given in “fix_qeq_reax.cpp”

Can someone explain us more detail for this value? I think that the factor conversion eV to Kcal/mol is 23.06 not 14.4!

Thanks for your help. Regards,


The reason why this value was chosen is that it is correct :-).

However, it does have an incorrect name, which is not a bug, just

1/14.4 is the value of the dielectric constant when charge is in electron
charges, distance is in angstroms, and energy is in electron volts. So a
better name would be QQR_TO_EV. The corresponding value when energy is in
kilocalories per mole is 1/332.06, and the ratio of these two is 23.06.

The electrostatic energy minimized by fix_qeq_reaxc.cpp is calcualted in
units of electron volts, but everything else in pair_style reax/c is
calculated in units of kilocalories per mole. Hence the confusion.


$ grep -e '332\.' -e '14\.' -e '23\.' USER-REAXC/*

fix_qeq_reax.cpp:#define EV_TO_KCAL_PER_MOL 14.4
reaxc_defs.h:#define C_ele 332.06371
reaxc_defs.h:#define EV_to_KCALpMOL 14.400000 // ElectronVolt --> KCAL per
reaxc_defs.h:#define KCALpMOL_to_EV 23.02 // 23.060549 //KCAL per MOLe
--> ElectronVolt