Ž . Chemical Physics 232 1998 299–306 A density functional study of weakly bound hydrogen bonded complexes Asit K. Chandra, Minh Tho Nguyen Department of Chemistry, UniÕersity of LeuÕen, Celestijnenlaan 200F, B-3001 LeuÕen, Belgium Received 19 September 1997; in final form 16 March 1998 Abstract Ž . Ž . Density functional theory DFT calculations with B3LYP exchange-correlation functional and using 6-31 qqG d,p Ž . basis functions have been performed on weakly bound hydrogen bonded complexes between HX X sF,Cl and alkenes and Ž . alkynes, such as C H , C HX X sH,F,Cl , C H and allene. Calculations have also been carried out at MP2 sfull level 2 4 2 4 2 of theory and using the same basis set as mentioned above for comparison with the DFT results. It has been observed that the BSSE uncorrected binding energies obtained from the B3LYP calculations are always lower than the corresponding MP2 results whereas opposite trend has been observed after BSSE correction. Hydrogen bond lengths obtained from MP2 and B3LYP calculations differ insignificantly. The H-X frequency shift due to complex formation has been well reproduced by the B3LYP method. q 1998 Elsevier Science B.V. All rights reserved. 1. Introduction Hydrogen bonding is an area of significant re- search interest owing to its physical and biological w x importance 1–4 . Theoretical study on weakly bound hydrogen bonded complexes is important for the clearer understanding of the nature of interaction involved, which on turn is crucial for the rationaliza- tion of structure. However, high level of quantum chemical calculations with electron correlation and large basis set are necessary for the proper descrip- tion of a weakly bound hydrogen bonded system. Electron correlation is very important in these sys- tems to take into account the dispersion forces. Ž . Density functional theory DFT provides an easy and expedient way to include electron correlation effects in ground-state molecules at reasonable com- putational cost. The purpose of the present work is to explore the performance of density functional proce- dures in the study of weak hydrogen bond involving p-electrons. There is a growing interest on the applicability of density functional procedures on real chemical prob- lems. It is well known that density functional theory Ž . DFT at local level of theory is seriously deficient and often overestimates the correlation energy as w x much as 100% 5,6 and thus not suitable for the study of weak interactions. However, opinions are diverse at non-local DFT level when one uses the Ž . gradient corrected exchange correlation XC func- w x tional 7–9 . Although exact density functional the- ory includes all correlation effects, including the dispersion force, the exact form of the functional in terms of electron density is not yet known. Thus success of a DFT calculation largely depends on the choice of an XC functional. Now-a-days DFT calcu- 0301-0104r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII: S0301-0104 98 00111-6