The possibility of an A-site (La3+/Sr2+) ordering in a colossal magnetoresistance manganite La3/4Sr1/4MnO 3 was explored using molecular dynamics (MD) simulations with a newly developed force field and quantum mechanics (QM) calculations on the structures obtained from MD. The calculated degrees of stabilization (enthalpy gain) of various patterns of A-site ordering are not significant enough to overcome the accompanying entropy loss, supporting the random A-site distribution in La 3/4Sr1/4MnO3. This approach combining MD and QM as well as the versatile force field developed in this study should be useful to investigate the structures and functions of magnetic tunnel junction devices involving mixed-valence manganites.