EN
The paper presents results of the ab initio electronic structure calculations performed for the (Fe_{0.5}Mn_{0.5})_2P_{0.67}Ge_{0.33} alloy, a member of (Fe_{1 - x}Mn_{x})_2P_{1 - y}Ge_{y} family of alloys showing a giant magnetocaloric effect. Calculations confirmed the strong relationship between the magnetic state and crystal structure of the alloy. To investigate the isostructural phase transition driven by external magnetic field observed in (Fe_{0.9}Mn_{1.1})P_{0.8}Ge_{0.2} we utilized the fixed spin moment approach. Total energy analysis confirmed the occurrence of isostructural phase transition.