Cannot declare multiple elements

[daniel-sintef]

Latest version of n2p2 does not allow users to map to more atoms than are declared in the nnp.weights. For example in the lammps package: /lammps/examples/PACKAGES/hdnnp

Modifying the pair_coef to the following:

pair_coeff * * H O O

(And also adding an extra mass to the data.H2O-360mol data file)

Masses 
1 1.00794
2 15.9994
3 15.9994

Causes the following error:

ERROR: Element mapping is inconsistent, NNP elements: 2, emap elements: 3.

I think this error no longer makes sense anymore, as it implies the user cannot specify more mappings than there are elements. This is annoying for user cases where we wish to divide types into different regions, e.g., H H H O O O for H/O in top, middle, bottom regions respectively.