The great sensitivity of ferroelectrics to defects, chemical bonding and pressure arises from a delicate balance between long-range Coulomb forces and short-range repulsions. To model the appearance of ferroelectricity, total-energy techniques are required which incorporate the effects of charge distortion and covalency. The paper presents the results of the investigations of electronic potential dependencies upon normal co-ordinates of B1u mode in large temperature range for the Sb2S3 crystal considered as week ferroelectric. The results revealed high anharmonicity of some atoms in this highly covalent semiconductive compound and B1u-mode polar distortion with temperature leading to appearance of week ferroelectricity.