  A previous study of diatomic molecules revealed that variational second-order
density matrix theory has serious problems in the dissociation limit when the
N-representability is imposed at the level of the usual two-index (P, Q, G) or
even three-index (T1, T2) conditions [H. van Aggelen et al., Phys. Chem. Chem.
Phys. 11, 5558 (2009)]. Heteronuclear molecules tend to dissociate into
fractionally charged atoms. In this paper we introduce a general class of
N-representability conditions, called subsystem constraints, and show that they
cure the dissociation problem at little additional computational cost. As a
numerical example the singlet potential energy surface of BeB+ is studied. The
extension to polyatomic molecules, where more subsystem choices can be
identified, is also discussed.
