In this study, we report a combined experimental and theoretical study on 3,5-bis(4-methoxyphenyl)-4,5-dihydro-1H-pyrazole-1-carbothioic 0-acid (C18H18N2O3S) molecule. The compound crystallizes in the trigonal space group R-3 with a = b = 27.7151(12) angstrom, c = 12.4866(6) angstrom. alpha = beta = 90.0 degrees, gamma = 120.00 degrees and Z = 18. The crystal packing is stabilized by O-H center dot center dot center dot O and O-H center dot center dot center dot S intermolecular hydrogen bonds. These hydrogen bond interactions are also proved by NBO analysis. A detailed spectroscopic investigation is performed by the application of FT-IR and FT-NMR in addition to the theoretical approaches. Small energy gap between the frontier molecular orbitals is responsible for the nonlinear optical activity of the title molecule. (C) 2014 Elsevier B.V. All rights reserved.