I think a "dcd" file can be used, provided that it is a restart file and not some other kind of file. For example, a coordinate trajectory file (or a blo.ip file as it could be called) only contains coordinates. Since F=ma is a second order differential equation more information is required for a proper restart; this information is contained in the restart file (the spru.nk file).

Lennart Nilsson
Karolinska Institutet
Stockholm, Sweden