We have investigated the structural, electronic and optical properties for two phases of polymorphs of hexamethylenetetraminium 2,4-dinitrophenolate monohydrate (C6H13N4+ center dot C6H3N2O5- center dot H2O) compound which were synthesized by Hoong-Kun Fun's group. The first phase possesess monoclinic P2(1)/m space group, while the other phase has triclinic P1 space group. The all electron full potential linearized augmented plane wave (FP-LAPW + lo) method within the local density approximation (LDA) and the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) were used. Calculation shows that the conduction band minimum (CBM) for the two phases is located between D and Z points of the Brillouin zone (BZ) while the valence band maximum (VBM) is located at the center of the"BZ, resulting in an indirect band gap. It has been found that the monoclinic (300 K) polymorph possesses a band gap of about 1.884 (LDA) and 1.935 (PBE-GGA) eV while triclinic (143 K) polymorph has a band gap of about 1.720 (LDA) and 1.773 (PBE-GGA) eV. For a deeper insight into the electronic structure, we have performed comprehensive optical properties calculations. These confirm the band gap reduction during the phase transition (300 K -> 143 K). The bond lengths and angles are calculated and compared with the experimental data; good agreement was found which reveals the accuracy of the calculations. (C) 2016 Elsevier B.V. All rights reserved.