# Sundials solver options question, is dt ignored(initial step), and is maxorder a viable option? What is the default, 4?

**URL:** <https://discourse.julialang.org/t/sundials-solver-options-question-is-dt-ignored-initial-step-and-is-maxorder-a-viable-option-what-is-the-default-4/47549>\
**Category:** General Usage\
**Tags:** ode\
**Created:** [September 30, 2020, 6:38pm UTC](https://discourse.julialang.org/t/sundials-solver-options-question-is-dt-ignored-initial-step-and-is-maxorder-a-viable-option-what-is-the-default-4/47549 "2020-09-30T18:38:14Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![cbasile21](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cbasile21/32/13072_2.png) [@cbasile21](https://discourse.julialang.org/u/cbasile21)\
**Post date:** [September 30, 2020, 6:38pm UTC](https://discourse.julialang.org/t/sundials-solver-options-question-is-dt-ignored-initial-step-and-is-maxorder-a-viable-option-what-is-the-default-4/47549/1 "2020-09-30T18:38:14Z")

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The solver compatibility chart, says that dt is ignored for sundials, is this correct? I did not see any errors when using it. Moreover, is maxorder a viable option? Is the default for CVODE\_BDF 4 or 5? Can it be changed? If predicted, can we somehow find what was the decided value? If maxorder is not a viable option, is there any advise to reach to similar results?

Thanks

Christian

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**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [September 30, 2020, 7:00pm UTC](https://discourse.julialang.org/t/sundials-solver-options-question-is-dt-ignored-initial-step-and-is-maxorder-a-viable-option-what-is-the-default-4/47549/2 "2020-09-30T19:00:21Z")

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> [@cbasile21](#):
>
> Moreover, is maxorder a viable option? Is the default for CVODE\_BDF 4 or 5? Can it be changed?

It’s 5 IIRC and it’s part of the algorithm kwargs as documented in [https://diffeq.sciml.ai/latest/solvers/ode\_solve/#ode\_solve\_sundials](https://diffeq.sciml.ai/latest/solvers/ode_solve/#ode_solve_sundials) .

> [@cbasile21](#):
>
> The solver compatibility chart, says that dt is ignored for sundials, is this correct? I did not see any errors when using it.

it sets the initial dt.

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<div class="post-metadata">

**Author:** ![cbasile21](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cbasile21/32/13072_2.png) [@cbasile21](https://discourse.julialang.org/u/cbasile21)\
**Post date:** [September 30, 2020, 7:22pm UTC](https://discourse.julialang.org/t/sundials-solver-options-question-is-dt-ignored-initial-step-and-is-maxorder-a-viable-option-what-is-the-default-4/47549/3 "2020-09-30T19:22:44Z")

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Thank you Chris, I was using maxorder instead of max\_order hoping that fixes the problem. And, yeah I do see it in the documentation , I did miss that , but other documentation that did not mention maxorder, or one that even wrote it as maxorder , rather than max\_order. Also, I thought I saw a missing x , in dt for sundials. Many thanks!

Christian