## modfile.default() — generate an ‘automatic’ filename

`default(root_name='undf', file_id='X', id1=1, id2=1, file_ext='')`

> This returns a generated file name, as `(root_name)(file_id)(id1)(id2)(file_ext)`, where id1 and id2 are printed as 4-digit numbers, padded with zeroes if necessary. For example, `'2ptn.B99990001.pdb'` results from root_name = `'2ptn'`, file_id = `'.B'`, id1 = 9999, id2 = 1, and file_ext = `'.pdb'`. This mimics the ‘automatic’ filename generation of previous versions of MODELLER when filenames were set to `'default'` or `'${DEFAULT}'`.