Enhancement-881: xmu outside [0, 0.5] is refused — xmu=2 turned an answer of 3.757e-7 into −2.53e-7 without a word

Enhancement-881: xmu outside [0, 0.5] is refused — xmu=2 turned an answer of 3.757e-7 into −2.53e-7 without a word

Scope: D1 of the
transient stress campaign of 2026-10-12.

ngspice: spicelib/analysis/cktsopt.c, CKTsetOpt: OPT_XMU is range-checked. Stock option.

examples/transopts_examples/ (new, shared with E-882 and
E-883). ngspice only.

Suites:

• transopts_examples: [1] and [2] are this enhancement's,
  7 of 7 per solver. On the E-875 binaries [1] fails; [2] is a control.
• The full sweep: 585 of 585.

What was wrong

xmu weights the trapezoidal rule (0.5) against backward Euler (0). On an RC with an inductor
whose answer at 4 µs is 3.757e-7:

• .option xmu=-1 gave 2.58e-6;
• .option xmu=2 gave −2.53e-7.

Neither printed anything. Outside [0, 0.5] the formula is neither method. Every sibling
tolerance has been range-checked since Enhancement-426 and
Enhancement-447.

The change

A value outside [0, 0.5] is refused, and the run keeps the value it had:

Warning -- Option xmu = 2 must lie in [0, 0.5]; ignored, keeping 0.5

Both decks now give 3.757e-7. xmu=0 and xmu=0.3 are taken as before.

The checks

transopts_examples:

• [1] xmu=-1 and xmu=2 are refused with a warning, and the run is the default run.
• [2] (control) xmu=0 and xmu=0.3 are taken, and move the answer.

Write-up: enhancements_doc/Enhancement-881.md