Ab initio studies of crystal field effects. VII. Structure of 2,3‐diketopiperazine using a 13‐molecule cluster, a calculation involving 1092 basis functions

The structure of 2,3‐diketopiperazine in a crystal phase (P21/c), modeled by a 13‐molecule cluster surrounded by point charges, was completely optimized using standard gradient procedures. The (ab initio) MIA approach was used to perform the SCF step, which using a 4‐21G basis set, involves calculations using 1092 basis functions. Results are in very good agreement with experimental (X‐ray) data, in contrast to a previous study in which a model was used consisting of only point charges.