The natural modes of the structure are calculated on the undamped system by determining the roots of the homogeneous equation system [K]*u – ω²*[M]*u = 0. A subspace iteration scheme is used to find the eigenvalues of this equation system and thus the natural frequencies ω and relevant displacement directions for computing the mode shapes.

Further also the mass matrix is required, representing the vibrating masses of the structure, as governing parameter of the Eigen value calculation. In the program, the self-weights and superimposed dead loads as defined for the static load-case calculation and any further user defined masses are considered for calculating a consistent mass matrix.