# Problems

Commonly encountered problems

1. Singularity during integration (IFAIL nonzero) often means either the equations are singular for your chosen parameter set, or you are using an inappropriate method, or else you are asking for a precision-level that cannot be achieved.
   In such cases you should inspect the w_deqsol.txt and iterate.txt files, or nagifail.txt if using the NAG library.
2. Slow integration suggests you may be using a wrong method.
   The BDF method is faster with a supplied Jacobian but, if this is not happening, your Jacobian is probably wrong.
3. Curve fitting is always slow but, if it takes much longer than the test files, you are probably using poor starting estimates.
4. Curve fitting leading to a very poor or strange looking fit is usually caused by one or more of the following.
   - You are not using a correct set of differential equations.
   - The differential equations are not formulated correctly.
   - You have senseless initial condition/parameter-estimates.
   - You are not using the right method or control parameters.
   - You are not using the correct transformation matrix A.
   - The integration procedure cannot function correctly causing an exit with the initial parameters unchanged.

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