The potential energy surfaces (PES), molecular and vibrational structure of hypophosphours acid (HPA) were investigated by HF, MP2 and DFT-B3LYP level of theory using 6-31G** basis set. In order to approach solid state spectra we optimized mono-, di-, tri-, tetra- and penta-mer structures and computed frequencies and intensities of the vibrational modes of VCD and IR. It is found that by increasing the number of HPA molecules in the chain, some of calculated vibrational frequencies approach to the experimental solid state values. The potential energy surfaces of HPA are calculated in a wide range on the plane perpendicular to the P-O bond where the hydrogen can rotate 360 degree through O=P-O-H dihedral angle. A circular valley is found going up and down on the plane, the valley is centered to the continuation of P-O bond. There are two minima at angles ±42.50 to the O=P-O plane giving two mirror conformers and one saddle point in between with a height of 284.87 cm-1 at the angles 0o and a complex barrier with a height of 2089.40 cm-1 at the angle 180o to the same plane. On top of the complex barrier, there is a small well with depth of 15.65 cm-1. To study the tunneling effect and pathway between the two conformers, the molecule is considered to have CS symmetry and a symmetric double minimum potential energy well with a barrier of 284.87 cm-1 height in the middle. With the constructed potential, the torsional motion of P-O-H (Hindered Rotation) of the monomer, using variation method and harmonic oscillator wave functions is studied and IR frequencies and relative intensities of vibrational modes are calculated. Due to the width of the well and the trigonometric shape of the barrier, tunneling can occur at the ground state

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