Volume 194, number 3 CHEMICAL PHYSICS LETTERS 26 June 1992 Water dimer properties in the gradient-corrected density functional theory K. Laasonen 1 IRRMA, Ecole Polytechnique F~d~ral de Lausanne CH- I O I 5 PHB-Ecublens, Switzerland F. Csajka and M. ParrineUo IBM Research Division, Zurich Research Laboratory, CH-8803 Riischlikon, Switzerland Received 17 January 1992; in final form 24 March 1992 Using a gradient-corrected version of the local density approximation to the energy density functional we calculate the struc- tural properties of the water dimer. We find that without gradient corrections the local density approximation gives realistic results for the intramolecular properties, but fails to give a correct description of the intermolecular interactions and the binding prop- erties. Such defects can be remedied with the inclusion of the gradient corrections. The resulting dimer properties are in good agreement with more sophisticated quantum chemical calculations. The calculation has been performed using a plane wave ex- pansion and a pseudopotential scheme. Thus it can be used to perform realistic Car-Parfinello-like molecular dynamics simula- tions of water. The problem of water-water interaction is of such relevance to so many fields of science that numerous experimental [ 1 ] and theoretical efforts have been devoted to it [ 2 ]. In its most simple form the prob- lem involves only two molecules. This is the reason why the study of the water dimer has received so much attention, making it probably the best under- stood hydrogen-bonded system. Given its limited size very accurate quantum chemical methods have been applied to describe such a system. These methods however cannot be applied to much larger clusters, nor is the possibility of using them in an ab initio molecular dynamics calculation realistic. For this reason we investigate here the ability of density func- tional schemes to describe the properties of water, this being an essential preliminary step towards a Car-ParrineUo-like simulation of bulk water. It is in fact known that while the local density approxima- tion (LDA) provides a good description of intra- Correspondence to: M. Parrinello, IBM Research Division, Zurich Research Laboratory, CH-8803 Riischlikon, Switzerland. t Present address: IBM Research Division, Zurich Research Laboratory, 8803 Riischlikon, Switzerland. molecular geometries [3 ] it is less suitable for de- scribing binding energies and the much weaker and subtle intermolecular interactions correctly. Here we also confirm this expected pattern for the case of the water dimer and then demonstrate that many of these difficulties can be overcome by using a gradient-cor- rected (GC) LDA. We first describe the calculation performed with an LDA and then the more accurate GC-LDA cal- culation. Keeping future ab initio molecular dynam- ics applications in mind, we use a Car-Parrinello [4] approach to calculate the electronic properties. We expand the Kohn-Sham orbitals into plane waves (PW), whereby only the valence electrons are treated explicitly. The interaction with the O ionic cores is described by a fully nonlocal norm-conserving pseu- dopotential [5 ] while for H we use a simple local pseudopotential to avoid the singularity of the 1/r potential. We use the Perdew-Zunger [6 ] parame- trization (PW) of the exchange and correlation functional. Use of PW implies the imposition of pe- riodic boundary conditions. Thus in order to decou- pie each molecule from its periodically repeated im- ages we use a large cubic unit cell of size 16 au or a 172 0009-2614/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.