Theoretical calculations of molecular dipole moment, polarizability, and first hyperpolarizability of glycine–sodium nitrate J. Hernández-Paredes * , Daniel Glossman-Mitnik, A. Duarte-Moller, N. Flores-Holguín Centro de Investigación en Materiales Avanzados, S.C. (CIMAV), Physics and Chemistry of Materials, Miguel de Cervantes Saavedra 120, Complejo industrial Chihuahua, Chihuahua 31109, México article info Article history: Received 5 November 2008 Received in revised form 12 March 2009 Accepted 12 March 2009 Available online 24 March 2009 Keywords: Dipole moment Polarizability Hyperpolarizability abstract In order to calculate the energy gap, molecular dipole moment, polarizability, and first hyperpolarizabil- ity of glycine–sodium nitrate (Na(NO 3 )C 2 H 5 NO 2 ), a series of basis sets including polarized and diffuse functions have been employed at the framework of: Hartree–Fock, Density Functional Theory, and Möller–Plesset Perturbation Theory methods. Geometry optimization was carried out with DFT-B3LYP 6-311++G(d,p). The geometrical differences between the optimized molecule and the molecule in solid phase were attributed to intramolecular and intermolecular forces that are present in solid phase. In addition, the results have revealed that hydrogen bonds not only play an important role determining the crystal structure of glycine–sodium nitrate but also decreasing its energy gap. Further, it was con- firmed that glycine–sodium nitrate has absolute value of dipole moment which is mainly caused by both the glycine dipolar character and the molecular geometry. Likewise, the calculations gave non-zero val- ues of polarizability and first hyperpolarizability which are related to the linear and nonlinear responses, respectively. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction In order to allow the second harmonic generation (SHG) pro- cess, the materials must possess an absolute value of the suscepti- bility (v (2) ) which is a bulk property and generally is associated with non-centrosymmetric crystalline structures. From a micro- scopic point of view, the description of the bulk property (v (2) ) is analogous to the molecular response named first hyperpolarizabil- ity (b). It is common that some organic molecules own absolute b val- ues. What is more, some of these molecules tend to crystallize in non-centrosymmetric crystalline structures; therefore efficient SHG process is achieved in these materials. As a result, photonic technologies have created an interest not only for the traditional inorganic materials but also organic materials for nonlinear optical (NLO) applications. Materials based on aminoacids have already proved to achieve SHG when they were irradiated by a Nd:YAG laser source (k  1064 nm) [1–5]. For example, some molecular crystals based on glycine have showed this phenomenon [6–10]. In particular, glycine–sodium nitrate (GSN) showed efficient SHG [6]. Besides, from ab-initio calculations some of its physical and chemical prop- erties were determined [11–13]. Ab-initio calculations are excellent alternative methods in the design of NLO molecules and help to predict some properties of the new materials, such as molecular dipole moments, polarizabil- ities, and hyperpolarizabilities [14–18]. However, in order to ob- tain reasonable NLO properties it is necessary to use a sequence of basis sets and, at a given level of theory the convergence can be achieved. Nevertheless, the basis sets which the convergence could be achieved are generally large. On the other hand, the Möl- ler–Plesset (MP2) methods are desirable for this purpose [19–20] but they are more computationally expensive. Consequently, NLO calculations have great computational cost due to the large basis sets employed plus the MP2 methods. On the basis depicted above, a sequence of diffuse and correla- tion consistent polarized valence basis sets were employed with the aim of calculate the energy gap, dipole moment, polarizability and first hyperpolarizability of GSN at Hartree–Fock (HF), Density Functional Theory (DFT) and MP2 methods. 2. Computational details Calculations were carried out using Gaussian 03W [21] package of programs. Gaussian 03W has already proved to be an important tool predicting molecular structures, spectroscopic properties, and molecular origins of NLO properties [22–26]. The single GSN molecule consists of a glycine molecule, a nitrate group and a sodium atom (Fig. 1). Its atomic positions, 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.03.014 * Corresponding author. Tel.: +52 614 4394842; fax: +52 614 4394852. E-mail addresses: jahernandezparedes@gmail.com (J. Hernández-Paredes), daniel. glossman@cimav.edu.mx (D. Glossman-Mitnik). Journal of Molecular Structure: THEOCHEM 905 (2009) 76–80 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem