Why is MATRIX27 mass matrix causing negative definite errors? The mass matrix was created by an external code to simulate surrounding water, and for certain degrees of freedom (DOF), the diagonal is zero.

ANSYS does not allow the diagonal term of a MATRIX27 to be zero and is replacing it with 1e-29. This is so small relative to the very large off-diagonal mass value that it produces the negative definite error.

Defect 68474 was filed to report this, but it was dispositioned as a limitation. A diagonal value within some reasonable number of orders of magnitude of the off-diagonal term must be specified.

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