Theor Chem Acc (2007) 117:885–901 DOI 10.1007/s00214-006-0209-y REGULAR ARTICLE Structure and dynamics of mesogens using intermolecular potentials derived from ab initio calculations Claudio Amovilli · Ivo Cacelli · Giorgio Cinacchi · Luca De Gaetani · Giacomo Prampolini · Alessandro Tani Received: 16 June 2006 / Accepted: 13 October 2006 / Published online: 19 December 2006 © Springer-Verlag 2006 Abstract A method for the calculation of the two- body intermolecular potential which can be applied to large molecules is presented. Each monomer is frag- mented in a number of moieties whose interaction ener- gies are used to recover the interaction energy of the whole dimer. For these reasons this strategy has been called fragmentation reconstruction method (FRM). By a judicious choice of the fragmentation scheme it is shown that very accurate interaction energies can be obtained. The sampling of the potential energy sur- face of a dimer is then used to obtain intermolecular force fields at several levels of complexity, suitable to be employed in bulk phase computer simulations. Applica- tions are presented for benzene and for some mesogenic molecules which constitute the principal interest of the authors. A number of properties ranging from phase stability, thermodynamic quantities, orientational order parameter and collective dynamics properties are com- puted and discussed. Keywords Condensed matter · Simulation methods · Liquid crystals · ab initio force field 1 Introduction In the theoretical approach to material science and, in particular, to liquid crystal (LC) field, computer simula- tions methods such as Monte Carlo (MC) and molecular C. Amovilli · I. Cacelli (B ) · G. Cinacchi · L. De Gaetani · G. Prampolini · A. Tani Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Risorgimento 35, 56126 Pisa, Italy e-mail: ivo@dcci.unipi.it dynamics (MD) are by far the most employed [1–4]. The structural and dynamic macroscopic properties, obtained through simulations, are derived from the adoption of a model potential, which contains the description of the molecular framework and interactions. The strong dependence of LC phase stability from the molecular features [1, 5, 6] makes the understanding of the link between microscopic structure and macroscopic mes- ophase properties a challenge for the scientific commu- nity. This extraordinary sensitivity to the details of the molecular structure arises [2, 6] from a complex inter- play between energetic effects (as the molecular interac- tions: electrostatic, dispersive and inductive forces) and entropic ones (like positional, orientational and confor- mational distributions). From a thermodynamic point of view, the balance among these free energy terms is critical for the (meso-)phase stability. Small variations in the molecular framework can alter this delicate equi- librium, affecting significantly the phase diagram of the system. As a matter of fact, it is in the force-field (FF) specifi- cation that the microscopic interactions are introduced in the simulation method, and the chemical identity of the molecule finds a correspondence in the bulk ob- servables. In particular, in LC field the abovementioned delicate interplay between the forces governing the mesogenic properties calls for very specific molecular models. This poses some doubts on the reliability of a straightforward adoption of the most widely employed FF’s [7–10], since their extension to large LC forming molecules, can be done only invoking a high degree of transferability. Indeed, it has been pointed out that small differences in the molecular structure may pro- duce impressive variations in the macroscopic proper- ties, so that transferability must be used with caution.