DIADORIM: a Monte Carlo Program for liquid simulations including quantum mechanics and molecular mechanics (QM/MM) facilities: applications to liquid ethanol

Um programa computacional para a simulacao de liquidos e solucoes utilizando o Metodo de Monte Carlo com algoritmo de Metropolis (MMC) e apresentado. Neste programa a energia total de interacao e dividida em classica e quântica, QM/MM: a contribuicao classica e obtida utilizando campo de forca e a contribuicao quântica e calculada utilizando metodos de quimica quântica. As seguintes facilidades foram implementadas: (i) o programa MOPAC 6 foi incorporado como uma sub-rotina do programa principal; (ii) alternativamente, um banco de dados de energias previamente obtidas pode ser utilizado; (iii) Simulacoes nos ensembles isotermico e isobarico, NpT, e canonico, NVT, podem ser realizadas; (iv) a trajetoria obtida ao longo do processo de MMC pode ser armazenada em disco rigido, utilizando o formato xtc, para analise e visualizacao utilizando outros programas. O programa foi utilizado para calcular propriedades estruturais e termodinâmicas do liquido etanol no ensemble NpT a 1,0 atm e 298 K. Distâncias e ângulos de ligacao das moleculas de etanol foram mantidos constantes, mas amostragem do ângulo diedral definido pelos atomos H-O-C-C foi efetuada. Para calcular a energia interna em funcao deste ângulo foram utilizados os metodos AM1, PM3 e MNDO implementados no programa MOPAC 6. Calculos tambem foram efetuados utilizando energias obtidas com HF/6-31g*, MP2/6-31g* e B3LYP/6-31g*. Diferencas significativas foram observadas na distribuicao dos ângulos diedrais na fase liquida. Entretanto, distribuicoes de energia para as interacoes de pares sao muito semelhantes, o mesmo acontecendo com as distribuicoes radiais de pares. Resultados obtidos para a densidade e calor de vaporizacao do liquido estao em otimo acordo com dados experimentais. This paper presents a Metropolis Monte Carlo (MMC) computer program developed to simulate liquids and solutions including QM/MM facilities: the energy from intermolecular interactions is calculated with classical force field functions and the internal molecular energies are calculated using Quantum Chemistry methods. The following facilities were implemented: (i) the semiempirical MOPAC 6 quantum chemistry package was included as a subroutine of the main MMC simulation program; (ii) alternatively, an interface with an external data bank was developed to allow the use of energies previously obtained; (iii) calculation in NpT and NVT ensembles are available; (iv) the trajectory generated along the MMC sampling can be saved using standard xtc file format allowing trajectory visualization and data analysis using external programs. The program was used to calculate thermodynamical and structural properties of liquids ethanol (ET) in the NpT ensemble at 1.0 atm and 298 K. Bond angles and bond distances of ethanol molecules were kept constant but torsions along the dihedral angle defined by the H-O-C-C atoms were sampled in the simulation. QM/MM calculations were performed using the MOPAC AM1, PM3 and MNDO Hamiltonians to calculate the internal molecular energy. Calculations using internal energies obtained with OPLS-AA force field, ab initio HF/6-31g*, MP2/6-31g* and B3LYP/6-31g* calculations were performed. The dihedral angles distributions obtained with different methodologies reveal extraordinary qualitative and quantitative differences. However, the pair energies and radial distributions functions obtained with the different methodologies are in good agreement. The values calculated for the density and enthalpy of vaporization of liquid ethanol are in good agreement with experimental data.