A user's manual for RAND's digital computer program for solving complex chemical composition problems by determining the distribution of chemical species in multicompartmented systems that minimizes the free energy of the system while conserving the mass of each chemical component. The total program, containing more than 35 subprograms, is flexible. During a run, the user may alter the data — e.g., add extra amounts of some chemicals (stresses), thus changing the model's composition and creating a new problem. He may also control some constants, cause computation and printing of certain measures of amount of error, and request various special computations and messages. Finally, the program may be used by either experienced computer programmers or those unfamiliar with programming methods because, first, the user refers to the data element by easily remembered alphanumeric names and, second, he gives operation instructions by control cards containing one alphanumeric word.