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--- faq:cholesky_decomp_failed [2015/12/14 13:11] – created ibethune
+++ faq:cholesky_decomp_failed [2015/12/14 19:54] – oschuett
@@ Line 1: / Line 1: @@
 ====== How do I solve "CPASSERT failed" in cp_fm_cholesky.F? ======
+If you see the following error message (line number correct as of CP2K 2.7, Dec 2015), with a routine calling stack that includes ''prepare_preconditioner'', for example:
+<code>
+*******************************************************************************
+ *   ___                                                                       *
+ *  /   \                                                                      *
+ * [ABORT]                                                                     *
+ *  \___/                             CPASSERT failed                          *
+ *    |                                                                        *
+ *  O/|                                                                        *
+ * /| |                                                                        *
+ * / \                                                  fm/cp_fm_cholesky.F:94 *
+ *******************************************************************************
+ ===== Routine Calling Stack =====
+cp_fm_cholesky_decompose
+make_full_inverse_cholesky
+make_preconditioner
+prepare_preconditioner
+init_scf_loop
+scf_env_do_scf
+qs_energies
+qs_forces
+qs_mol_dyn_low
+CP2K
+</code>
+This says that CP2K failed to compute a Cholesky decomposition during the construction of the preconditioner for OT. Most likely, the overlap matrix (S) has become singular (or at least numerically close to singular).  Here are some causes / possible solutions:
+  * Try some of the other [[https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/DFT/SCF/OT.html#PRECONDITIONER|PRECONDITIONER]] options, which may prove to be more stable for your system.
+  * Decrease the value of [[https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/DFT/QS.html#list_EPS_DEFAULT|EPS_DEFAULT]] or [[https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/DFT/QS.html#list_EPS_PGF_ORB|EPS_PGF_ORB]], to reduce the amount of numerical noise in the construction of S.
+  * Check carefully that the Basis Set(s) chosen for your calculation are appropriate for the system.  If the basis set is overcomplete this could result in a singular overlap matrix - try switching to a smaller basis set.