Hi

I have two questions:

1. I calculated a set of descriptors using calculate molecular property for a set of ligands. O_count and N_count were two amongst them, and my interest was to compute the total number of nitrogens and oxygens in a molecule. so i used the basic  arithmetic module in small molecules--property calculation, but it shows an error as: Unrecognized error code from pipeline pilot:-968. How do i resolve it

2. If i want to introduce a new descriptor, how do i insert it in DS.