# Implicit \*DYNAMIC step after a strained step: internal energy (ELSE) and the step's "initial energy" start at twice the value, with nothing moving **URL:** \ **Category:** Analysis issues\ **Created:** [October 21, 2026, 7:00pm UTC](https://calculix.discourse.group/t/implicit-dynamic-step-after-a-strained-step-internal-energy-else-and-the-steps-initial-energy-start-at-twice-the-value-with-nothing-moving/4208 "2026-10-21T19:00:30Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Sandervdb](https://avatars.discourse-cdn.com/v4/letter/s/49beb7/32.png) [@Sandervdb](https://calculix.discourse.group/u/Sandervdb)\ **Post date:** [October 21, 2026, 7:00pm UTC](https://calculix.discourse.group/t/implicit-dynamic-step-after-a-strained-step-internal-energy-else-and-the-steps-initial-energy-start-at-twice-the-value-with-nothing-moving/4208/1 "2026-10-21T19:00:30Z")
## Summary At the start of an implicit `*DYNAMIC` step, the internal energy (`*EL PRINT` ELSE) is raised by the full strain energy of the current state. A cube carries a static load with a strain energy of 5.0E-04. A `*DYNAMIC` step then holds the same load, and nothing moves. ELSE is 1.0E-03 from the first increment on, and so is the “initial energy (at start of step)” that ccx prints for its energy balance. A second `*DYNAMIC` step adds the same amount again (1.5E-03). If the load is released instead, ELSE oscillates between 5.0E-04 and 1.0E-03, and ELSE + ELKE stays at 1.0E-03, twice the energy the cube holds. The equivalent deck with a second `*STATIC` step keeps ELSE at 5.0E-04. A `*DYNAMIC` first step starts from zero. Section 7.53 (\*EL PRINT) of the 2.23 manual says that in nonlinear calculations “the internal energy at any time depends on the accumulated energy at all previous times”. With no motion and no work done, that energy cannot change. The step after an NLGEOM static step (`static_dyn.inp` with `*STEP, NLGEOM` on step 1) is doubled in the same way (4.99E-04 to 9.98E-04). ## Version ccx 2.23 (macOS arm64). Source: master 9ba5d76 (16 Oct 2026). ## Already reported or fixed? Not reported: I found no matching GitHub issue (open or closed) or pull request in Dhondtguido/CalculiX, and no calculix.discourse.group topic (searched 21 Oct 2026). Not fixed: `nonlingeo.c` and `resultsmech.f` are unchanged on master (9ba5d76). Related: - [BEAM32R - Strange internal energy values](https://calculix.discourse.group/t/2731) (B32R internal energy values near 1e-313): also about the internal energy, but for beam elements in a single step, a different problem. ## Minimal input `static_dyn.inp` is one C3D8 cube. Only the z-displacements of the four top nodes are free. E = 1000, nu = 0, so k = 1000, and 4 x 0.25 on the top gives u = 1.0E-03 and a strain energy of k u^2 / 2 = 5.0E-04. Step 2 keeps the load (carried over) and runs ten increments of the dynamic step. ```auto *NODE 1, 0., 0., 0. 2, 1., 0., 0. 3, 1., 1., 0. 4, 0., 1., 0. 5, 0., 0., 1. 6, 1., 0., 1. 7, 1., 1., 1. 8, 0., 1., 1. *ELEMENT, TYPE=C3D8, ELSET=EALL 1, 1, 2, 3, 4, 5, 6, 7, 8 *NSET, NSET=BOT 1, 2, 3, 4 *NSET, NSET=TOP 5, 6, 7, 8 *MATERIAL, NAME=M *ELASTIC 1000., 0. *DENSITY 0.3 *SOLID SECTION, ELSET=EALL, MATERIAL=M *BOUNDARY BOT, 1, 3 TOP, 1, 2 *STEP *STATIC *CLOAD TOP, 3, 0.25 *EL PRINT, ELSET=EALL ELSE *END STEP *STEP *DYNAMIC, DIRECT 2.51327412287E-4, 2.51327412287E-3 *END STEP ``` Variants: - `static_static.inp`: step 2 is `*STATIC` instead of `*DYNAMIC, DIRECT` with its data line. - `static_dyn_dyn.inp`: `static_dyn.inp` with a copy of step 2 appended as step 3. - `static_release.inp`: step 2 is replaced by this step, which removes the load and runs one period (250 increments): ```auto *STEP, INC=1000 *DYNAMIC, DIRECT 2.51327412287E-4, 0.0628318530718 *CLOAD, OP=NEW *EL PRINT, ELSET=EALL, FREQUENCY=25 ELSE, ELKE *END STEP ``` ## How to run `ccx static_dyn` (and the variants the same way) ## Expected ELSE stays 5.000000E-04 in step 2 of `static_dyn.inp`, as in step 2 of `static_static.inp`: nothing moves and no work is done. In `static_release.inp`, ELSE + ELKE = 5.0E-04 at all times, with ELSE going from 5.0E-04 to 0 and back. ## Actual `static_dyn.dat`: ```auto internal energy (element, energy) for set EALL and time 0.1000000E+01 1 5.000000E-04 ... internal energy (element, energy) for set EALL and time 0.1000251E+01 1 1.000000E-03 ``` ELSE stays at 1.000000E-03 up to the end of step 2. The displacement does not change. ccx prints on stdout for step 2: ```auto initial energy (at start of step) = 1.000000e-03 ... internal energy = 1.000000e-03 kinetic energy = 3.892867e-39 ``` `static_static.inp`: 5.000000E-04 in both steps. `static_dyn_dyn.inp`: 5.0E-04, 1.0E-03, 1.5E-03 in steps 1 to 3. `static_release.inp` (ELSE, ELKE, sum): ```auto time 1.006283 8.272721E-04 1.727255E-04 9.999976E-04 time 1.012566 5.477679E-04 4.522258E-04 9.999937E-04 time 1.031416 9.999997E-04 1.769592E-11 9.999997E-04 ``` At every output time, ELKE + (ELSE - 5.0E-04) = 5.0E-04. So the kinetic energy is right, and ELSE carries an extra 5.0E-04 throughout. ## Where it goes wrong `nonlingeo.c` allocates the start-of-increment fields at the start of the step. `stiini` and `emeini` are zero (lines 846-847), and `enerini` is a copy of `ener`, the energy at the end of the previous step (lines 849-856). For `*DYNAMIC` the initial acceleration block follows (line 909: `if((*nmethod==4)&&(*ithermal!=2)&&(icfd==0)){`). In it, `results()` is called at line 1497 with `iout=0` (set at line 1015), passing `stiini`, `emeini` and `enerini`. With `nener==1` and `iout==0`, `resultsmech.f` updates the energy (lines 1049-1073): ```fortran ! sigma.d(epsilon)=(sigma_ini+sigma)*(epsilon-epsilon_ini)/2 ener(1,jj,i)=enerini(1,jj,i)+ & ((emec(1)-emeini(1,jj,i))* & (stre(1)+stiini(1,jj,i))+ ``` With `emeini = 0` and `stiini = 0`, this adds `emec*stre/2`, the full strain energy of the current state, to the energy of the previous step. `stiini` and `emeini` only get the values of `sti` and `eme` at the start of the first increment (lines 1640-1641). By then `ener` already holds the doubled value, and the first increment copies it into `enerini`. The update in the initial-acceleration pass does not hinge on the `((iout.eq.0).and.(nener.eq.1))` term added on 07.07.2023 (`resultsmech.f` line 1057, LOGBOOK “energy was not calculated in an increment for which no printing output was requested”). `*DYNAMIC` sets `iperturb(1)=2` (`dynamics.f` line 68), so the older term `(nmethod.eq.4).and.((iperturb(1).gt.1).and.(nlgeom_undo.eq.0))` (lines 1058-1060) already lets this pass through. Removing the 2023 term alone would not fix it. ## Fix After allocating `stiini` and `emeini` in `nonlingeo.c` (lines 846-847), copy `sti` and `eme` into them, as lines 1640-1641 do at the start of each increment. Then the energy update in the initial-acceleration pass adds `(emec-eme)*(stre+sti)/2`, which is zero for an unchanged state. Not built or tested here. ---
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [October 21, 2026, 8:12pm UTC](https://calculix.discourse.group/t/implicit-dynamic-step-after-a-strained-step-internal-energy-else-and-the-steps-initial-energy-start-at-twice-the-value-with-nothing-moving/4208/2 "2026-10-21T20:12:57Z")
Confirmed (using official build of ccx 2.23 on Windows), needs a GitHub ticket. static\_dyn.dat: ```auto S T E P 1 INCREMENT 1 internal energy (element, energy) for set EALL and time 0.1000000E+01 1 5.000000E-04 S T E P 2 INCREMENT 1 internal energy (element, energy) for set EALL and time 0.1000251E+01 1 1.000000E-03 INCREMENT 2 internal energy (element, energy) for set EALL and time 0.1000503E+01 1 1.000000E-03 INCREMENT 3 internal energy (element, energy) for set EALL and time 0.1000754E+01 1 1.000000E-03 INCREMENT 4 internal energy (element, energy) for set EALL and time 0.1001005E+01 1 1.000000E-03 INCREMENT 5 internal energy (element, energy) for set EALL and time 0.1001257E+01 1 1.000000E-03 INCREMENT 6 internal energy (element, energy) for set EALL and time 0.1001508E+01 1 1.000000E-03 INCREMENT 7 internal energy (element, energy) for set EALL and time 0.1001759E+01 1 1.000000E-03 INCREMENT 8 internal energy (element, energy) for set EALL and time 0.1002011E+01 1 1.000000E-03 INCREMENT 9 internal energy (element, energy) for set EALL and time 0.1002262E+01 1 1.000000E-03 INCREMENT 10 internal energy (element, energy) for set EALL and time 0.1002513E+01 1 1.000000E-03 ``` ---
**Author:** ![Sandervdb](https://avatars.discourse-cdn.com/v4/letter/s/49beb7/32.png) [@Sandervdb](https://calculix.discourse.group/u/Sandervdb)\ **Post date:** [October 22, 2026, 7:22am UTC](https://calculix.discourse.group/t/implicit-dynamic-step-after-a-strained-step-internal-energy-else-and-the-steps-initial-energy-start-at-twice-the-value-with-nothing-moving/4208/3 "2026-10-22T07:22:52Z")
Posted: [Implicit \*DYNAMIC step after a strained step: internal energy (ELSE) and the step's "initial energy" start at twice the value, with nothing moving · Issue #232 · Dhondtguido/CalculiX · GitHub](https://github.com/Dhondtguido/CalculiX/issues/232)