Six conformers of benzothiadiazole (BTD) flanked by two thiophenes on each side (T4) and mono-functionalized by phosphonic acid (A) (T4-BTDA) have been investigated by Density Functional Theory (DFT). The performance of six different functionals, namely Becke's three-parameter Lee–Yang–Parr (B3LYP) and B3PW91 with (Perdew and Wang correlation), BHandH, M06-2X, Coulomb-attenuating-method-B3LYP (CAM-B3LYP), and Grimme's D2 dispersion model (ωB97XD) in combination with four different basis sets of Pople's 6-31G(d), 6-31+G(d), 6-311G(d), and 6-311+G(d) have been assessed for the prediction of Frontier Molecular Orbitals (FMOs) and the maximum of absorption spectra in comparison with available experimental data. Computed results reveal that the most stable conformer in the solvent is favored by the sulfur–nitrogen (S–N) non-covalent interaction and the orientation of thiophene. Besides, it has been revealed that the anchoring group stabilizes the conformer when sulfur and oxygen of the phosphoryl group (PO) are in transposition. Regarding the available electronics experimental data of the studied dye, calculations showed that the B3PW91/6-311G(d) method was the right choice to obtain FMO energies, while the TD-BHandH/6-311G(d) method was suitable for simulating the spectral absorption for the studied dye. Using BHandH functional and (6-311G(d), LANL2DZ) basis sets, the adsorption energy on titania anatase 101 with a (TiO₂)₉ slab and the photovoltaic performance of studied dyes have been reported. The calculated cell performance was very close to the experimental value. This indicates that the calculation schema is consistent.