Summation of van der Waals (VDW) atomic radii is commonly used as a criterion for whether donor and acceptor atoms, such as hydrogen and oxygen, nitrogen or fluorine, are close enough to form a hydrogen bond, being applied, together with a geometric constraint, as part of the pararneterisation in determining whether or not a particular force-field 'detects' a hydrogen bond. In general, VDW radii overestimate the donor-acceptor separation at which hydrogen bonding may be possible. A new method, based on electron density topology, is proposed which gives modified transferable atomic radii more in keeping with spectroscopic evidence. (c) 2006 Elsevier B.V. All rights reserved.