# Relative Tolerance

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 June 1, 2009, 22:31 Relative Tolerance #1 New Member   Rachel Harch Join Date: Mar 2009 Location: RAdelaide Posts: 2 Rep Power: 0 Hi I was wondering if someone could help me to understand the details of relTol. I am unsure which has the stricter convergence criteria relTol=0.1 or relTol=0.001. I understand that if you set relTol=0 that you force the solution to converge to the solver tolerance, but I am not sure about other values. Cheers Rach

 June 4, 2009, 10:56 #2 Senior Member   Rishi . Join Date: Mar 2009 Posts: 144 Rep Power: 10 Hello, relTol is a parameter to say how much the intial residual should reduce in a particular time step. e.g. relTol=0, then default tolerence is targetted. (e-6 here) DICPCG: Solving for p, Initial residual = 1, Final residual = 7.55402e-07, No Iterations 35 ... DICPCG: Solving for p, Initial residual = 0.379232, Final residual = 3.38648e-07, No Iterations 34 e.g. relTol=0.01, then solver only tries to reduce residual by certain factor. This relTol is the order of magnitude reduction. So here Solver tries to reach a value below 1/100 of initial value. DICPCG: Solving for p, Initial residual = 1, Final residual = 0.00837542, No Iterations 24 ... DICPCG: Solving for p, Initial residual = 0.378888, Final residual = 0.00355735, No Iterations 23 So relTol=0.001 is stricter criteria & requires more iterations. Observe the number of iterations required in the above examples, for the same calculation.

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