True energy of halogen bonding in dimers of bromine and iodine derivatives of trityl alcohols has been calculated by the M05-2X/6-311G(d,p) and MP2/6-311G(d,p) method using simple substitution scheme. This approach allows to separate energy of halogen bonds and general dispersion and electrostatic interactions. Results of calculations demonstrate that halogen bonding provides 30-100% contribution into total interaction energy between monomers depending on their mutual orientation. Increase of halogen bonding energy with elongation of the halogen center dot center dot center dot pi distance indicates that geometry of dimers in the crystal phase is not optimal for these interactions. (C) 2008 Elsevier B.V. All rights reserved.