Symmetric Matrices
Use the Matrix symmetry list (see the General section on the Settings window for Advanced for a solver node such as Stationary Solver). There you can explicitly state whether the assembled matrices (stiffness matrix, mass matrix) resulting from the compiled equations are symmetric or not.
Normally the matrices from a single-physics structural mechanics problem are symmetric, but there are exceptions, including the following cases:
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Multiphysics models solving for several physics simultaneously (for example, heat transfer and structural mechanics). Solving for several Structural Mechanics interfaces, such as shells combined with beams, does not create unsymmetrical matrices.
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Contact with friction.
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Elastoplastic analysis
Advanced and Stationary Solver in the COMSOL Multiphysics Reference Manual
One of the benefits of using the symmetric solvers is that they use less memory and are faster. The default option is Automatic, which means the solver automatically detects if the system is symmetric or not. Some solvers do not support symmetric matrices and always solve the full system regardless of symmetry.
Selecting the Symmetric option for a model with unsymmetric matrices may lead to incorrect results. For a nonlinear problem that is only weakly unsymmetric, it may still be useful, since the faster solution of the symmetric problem may offset the lower convergence rate. This is, for example, the case for contact problems with a low coefficient of friction.
Complex matrices can be unsymmetric, symmetric, or Hermitian. Hermitian matrices do not appear in structural mechanics problems.
Selecting the Hermitian option for a model with complex-valued symmetric matrices produces incorrect results.