Normal coordinate analysis and Nonlinear Optical Response of cross-conjugated system 4,4-Dimethyl Benzophenone M. Amalanathan a , T.S. Xavier a , I. Hubert Joe a,⇑ , V.K. Rastogi b a Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram-695 015, Kerala, India b Physics Department, CCS University Campus, Meerut, India highlights  DFT calculations have been performed on the NLO crystal 4,4- Dimethyl Benzophenone (4DMBP).  The NCA was performed on 4DMBP to reproduce its experimental harmonic vibrational wavenumbers excellently.  The IR and Raman spectra of the compound were analyzed.  The calculated first order hyperpolarizability of 4DMBP calculated and is found to be 1.97  10 30 e.s.u.  NBO, HOMO and LUMO analysis were also performed by DFT approach. graphical abstract article info Article history: Received 18 March 2013 Received in revised form 22 July 2013 Accepted 31 July 2013 Available online 7 August 2013 Keywords: NCA DFT Hyperpolarizability Vibrational spectra and THz spectra abstract FT-Raman and IR spectra of the nonlinear optical active crystal, 4,4-Dimethyl Benzophenone (4DMBP) have been recorded and analyzed The equilibrium geometry, harmonic vibrational wavenumbers of 4DMBP investigated with the help of density functional theory (DFT) method. The assignments of the vibrational spectra have been carried out with the help of normal coordinate analysis (NCA) following the scaled quantum mechanical force field methodology (SQMFF). The calculated hyperpolarizability value shows the nonlinear optical activity of the molecule. The value of HOMO–LUMO energy, Mulliken and the natural charges are calculated and analyzed. The Natural bond orbital analysis confirms the occurrence of intramolecular charge transfer interaction. Ó 2013 Elsevier B.V. All rights reserved. Introduction Organic materials with delocalised p electrons have attracted considerable research attention as nonlinear optical (NLO) materi- als in recent years with respect to their future potential applications in the field of optoelectronics and photonics, due to their large first hyperpolarizabilities b [1–4]. In order to improve the second-order NLO response at the macroscopic level, one must improve the NLO response at the molecular level [5]. Extensive literature is available on the organic NLO materials with deep theoretical and experimen- tal studies [6–10]. The NLO efficiency can be enhanced by increasing the extent of charge transfer across the molecule by plac- ing a strong electron donor and acceptor groups at the ends of the highly polarizable conjugated bridge. At the molecular level the hyperpolarizabilities b and c are responsible for the nonlinear opti- cal properties including second and third harmonic generation and electro-optic effect. Experimental measurements and theoretical calculations on molecular first hyperpolarizability b become one of the key factors in the second-order NLO materials design. Theoretical determination of hyperpolarizability is quite useful both in understanding the relationship between the molecular 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.07.100 ⇑ Corresponding author. Tel.: +91 471 2351053; fax: +91 471 2530023. E-mail address: hubertjoe@gmail.com (I. Hubert Joe). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 116 (2013) 574–583 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa