diff --git a/README.md b/README.md
index ac6e234..5e44762 100644
--- a/README.md
+++ b/README.md
@@ -1 +1,15 @@
-# Cardio
\ No newline at end of file
+# Cardio
+
+## ANSYS Transient Structural command snippet (direct solver)
+
+If you see a warning that an iterative solver was used, you can force a
+direct solver in Mechanical APDL with `EQSLV,SPARSE` before `SOLVE`:
+
+```apdl
+/SOLU
+ANTYPE,TRANS
+TRNOPT,FULL
+EQSLV,SPARSE
+SOLVE
+FINISH
+```
diff --git a/usermat_neohookean.f90 b/usermat_neohookean.f90
new file mode 100644
index 0000000..666331b
--- /dev/null
+++ b/usermat_neohookean.f90
@@ -0,0 +1,256 @@
+! ---------------------------------------------------------------------------
+! ANSYS USERMAT: Large-deformation Neo-Hookean (compressible penalty)
+! ---------------------------------------------------------------------------
+! This implementation follows the common USERMAT argument list used by ANSYS
+! Mechanical APDL when TB,USER is enabled with large-deformation kinematics.
+! If your ANSYS version uses a different interface, adjust the subroutine
+! signature accordingly.
+!
+! Material parameters (PROPS):
+!   PROPS(1) = shear modulus, mu
+!   PROPS(2) = volumetric penalty parameter, d
+!   PROPS(3) = optional ramp time for penalty (use 0 to disable)
+!
+! Notes:
+! - Compressible penalty formulation:
+!   W = (mu/2) * (I1 - 3) + (1/d) * (J - 1)^2
+!
+! Voigt order (ANSYS):
+!   STRESS(1:6) = [Sxx, Syy, Szz, Sxy, Syz, Sxz] (Cauchy)
+! ---------------------------------------------------------------------------
+      SUBROUTINE USERMAT(
+     &  STRESS,STATEV,DDSDDE,SSE,SPD,SCD,RPL,DDSDDT,DRPLDE,DRPLDT,
+     &  STRAN,DSTRAN,TIME,DTIME,TEMP,DTEMP,PREDEF,DPRED,CMNAME,
+     &  NDI,NSHR,NTENS,NSTATV,PROPS,NPROPS,COORDS,DROT,PNEWDT,
+     &  CELENT,DFGRD0,DFGRD1,NOEL,NPT,LAYER,KSPT,KSTEP,KINC )
+
+      IMPLICIT NONE
+
+!     Arguments
+      CHARACTER*80 CMNAME
+      INTEGER NDI,NSHR,NTENS,NSTATV,NPROPS
+      INTEGER NOEL,NPT,LAYER,KSPT,KSTEP,KINC
+      DOUBLE PRECISION STRESS(NTENS),STATEV(NSTATV),DDSDDE(NTENS,NTENS)
+      DOUBLE PRECISION SSE,SPD,SCD,RPL,DDSDDT(NTENS,NTENS)
+      DOUBLE PRECISION DRPLDE(NTENS),DRPLDT
+      DOUBLE PRECISION STRAN(NTENS),DSTRAN(NTENS)
+      DOUBLE PRECISION TIME(2),DTIME,TEMP,DTEMP,PREDEF,DPRED
+      DOUBLE PRECISION PROPS(NPROPS),COORDS(3),DROT(3,3),PNEWDT,CELENT
+      DOUBLE PRECISION DFGRD0(3,3),DFGRD1(3,3)
+
+!     Locals
+      DOUBLE PRECISION F(3,3),CMAT(3,3),I3(3,3)
+      DOUBLE PRECISION S2PK(3,3),CAUCHY(3,3)
+      DOUBLE PRECISION MU,DPEN,DPEN_EFF,J,TRC,PSI,RAMP
+      DOUBLE PRECISION CWORK(3,3),SPERT(3,3),SMINUS(3,3)
+      DOUBLE PRECISION EPS,DC,DE
+      DOUBLE PRECISION MAT_DET3
+      EXTERNAL MAT_INV3
+      INTEGER JIDX
+
+!     Initialize identity
+      CALL MAT_IDENTITY(I3)
+
+!     Deformation gradient at end of step
+      F = DFGRD1
+
+!     Right Cauchy-Green tensor: C = F^T * F
+      CALL MAT_MATMULT(TRANSPOSE(F), F, CMAT)
+
+!     Determinant of F
+      J = MAT_DET3(F)
+
+!     Material properties
+      MU = PROPS(1)
+      IF (NPROPS .GE. 2) THEN
+        DPEN = PROPS(2)
+      ELSE
+        DPEN = 1.0D-6 / MAX(MU, 1.0D-12)
+      END IF
+      RAMP = 1.0D0
+      IF (NPROPS .GE. 3) THEN
+        IF (PROPS(3) .GT. 0.0D0) THEN
+          RAMP = MIN(1.0D0, TIME(1) / PROPS(3))
+        END IF
+      END IF
+      DPEN_EFF = DPEN / MAX(RAMP, 1.0D-12)
+
+!     2nd PK stress (Total Lagrangian, penalty)
+!     S = mu * I + (2/d_eff) * (J - 1) * J * Cinv
+      CALL NEOHOOK_S2PK(CMAT, MU, DPEN_EFF, S2PK, TRC)
+
+!     Strain energy density
+      PSI = 0.5D0*MU*(TRC - 3.0D0) + (1.0D0/DPEN_EFF) * (J - 1.0D0)**2
+      SSE = PSI
+
+!     Cauchy stress: sigma = (1/J) * F * S * F^T
+      CALL MAT_MATMULT(F, S2PK, CWORK)
+      CALL MAT_MATMULT(CWORK, TRANSPOSE(F), CAUCHY)
+      CAUCHY = CAUCHY / J
+
+!     Fill STRESS (Voigt) with Cauchy stress
+      STRESS(1) = CAUCHY(1,1)
+      STRESS(2) = CAUCHY(2,2)
+      STRESS(3) = CAUCHY(3,3)
+      STRESS(4) = CAUCHY(1,2)
+      STRESS(5) = CAUCHY(2,3)
+      STRESS(6) = CAUCHY(1,3)
+
+!     Consistent tangent via numerical differentiation in Green-Lagrange strain
+      CALL ZERO_TANGENT(DDSDDE, NTENS)
+      EPS = 1.0D-8
+      DO JIDX = 1,6
+        DC = EPS
+        CWORK = CMAT
+        IF (JIDX .EQ. 1) THEN
+          CWORK(1,1) = CWORK(1,1) + DC
+        ELSEIF (JIDX .EQ. 2) THEN
+          CWORK(2,2) = CWORK(2,2) + DC
+        ELSEIF (JIDX .EQ. 3) THEN
+          CWORK(3,3) = CWORK(3,3) + DC
+        ELSEIF (JIDX .EQ. 4) THEN
+          CWORK(1,2) = CWORK(1,2) + DC
+          CWORK(2,1) = CWORK(2,1) + DC
+        ELSEIF (JIDX .EQ. 5) THEN
+          CWORK(2,3) = CWORK(2,3) + DC
+          CWORK(3,2) = CWORK(3,2) + DC
+        ELSE
+          CWORK(1,3) = CWORK(1,3) + DC
+          CWORK(3,1) = CWORK(3,1) + DC
+        END IF
+        CALL NEOHOOK_S2PK(CWORK, MU, DPEN_EFF, SPERT, TRC)
+
+        CWORK = CMAT
+        IF (JIDX .EQ. 1) THEN
+          CWORK(1,1) = CWORK(1,1) - DC
+        ELSEIF (JIDX .EQ. 2) THEN
+          CWORK(2,2) = CWORK(2,2) - DC
+        ELSEIF (JIDX .EQ. 3) THEN
+          CWORK(3,3) = CWORK(3,3) - DC
+        ELSEIF (JIDX .EQ. 4) THEN
+          CWORK(1,2) = CWORK(1,2) - DC
+          CWORK(2,1) = CWORK(2,1) - DC
+        ELSEIF (JIDX .EQ. 5) THEN
+          CWORK(2,3) = CWORK(2,3) - DC
+          CWORK(3,2) = CWORK(3,2) - DC
+        ELSE
+          CWORK(1,3) = CWORK(1,3) - DC
+          CWORK(3,1) = CWORK(3,1) - DC
+        END IF
+        CALL NEOHOOK_S2PK(CWORK, MU, DPEN_EFF, SMINUS, TRC)
+
+        IF (JIDX .GE. 4) THEN
+          DE = DC
+        ELSE
+          DE = 0.5D0 * DC
+        END IF
+        DDSDDE(1,JIDX) = (SPERT(1,1) - SMINUS(1,1)) / (2.0D0 * DE)
+        DDSDDE(2,JIDX) = (SPERT(2,2) - SMINUS(2,2)) / (2.0D0 * DE)
+        DDSDDE(3,JIDX) = (SPERT(3,3) - SMINUS(3,3)) / (2.0D0 * DE)
+        DDSDDE(4,JIDX) = (SPERT(1,2) - SMINUS(1,2)) / (2.0D0 * DE)
+        DDSDDE(5,JIDX) = (SPERT(2,3) - SMINUS(2,3)) / (2.0D0 * DE)
+        DDSDDE(6,JIDX) = (SPERT(1,3) - SMINUS(1,3)) / (2.0D0 * DE)
+      END DO
+      RETURN
+      END SUBROUTINE USERMAT
+
+! ---------------------------------------------------------------------------
+      SUBROUTINE MAT_IDENTITY(I3)
+      IMPLICIT NONE
+      DOUBLE PRECISION I3(3,3)
+      I3 = 0.0D0
+      I3(1,1) = 1.0D0
+      I3(2,2) = 1.0D0
+      I3(3,3) = 1.0D0
+      END SUBROUTINE MAT_IDENTITY
+
+! ---------------------------------------------------------------------------
+      SUBROUTINE MAT_MATMULT(A,B,C)
+      IMPLICIT NONE
+      DOUBLE PRECISION A(3,3),B(3,3),C(3,3)
+      INTEGER I,J,K
+      C = 0.0D0
+      DO I=1,3
+        DO J=1,3
+          DO K=1,3
+            C(I,J) = C(I,J) + A(I,K) * B(K,J)
+          END DO
+        END DO
+      END DO
+      END SUBROUTINE MAT_MATMULT
+
+! ---------------------------------------------------------------------------
+      SUBROUTINE MAT_INV3(A, AINV)
+      IMPLICIT NONE
+      DOUBLE PRECISION A(3,3),AINV(3,3)
+      DOUBLE PRECISION DET
+      DOUBLE PRECISION MAT_DET3
+
+      DET = MAT_DET3(A)
+      AINV(1,1) =  (A(2,2)*A(3,3)-A(2,3)*A(3,2)) / DET
+      AINV(1,2) = -(A(1,2)*A(3,3)-A(1,3)*A(3,2)) / DET
+      AINV(1,3) =  (A(1,2)*A(2,3)-A(1,3)*A(2,2)) / DET
+      AINV(2,1) = -(A(2,1)*A(3,3)-A(2,3)*A(3,1)) / DET
+      AINV(2,2) =  (A(1,1)*A(3,3)-A(1,3)*A(3,1)) / DET
+      AINV(2,3) = -(A(1,1)*A(2,3)-A(1,3)*A(2,1)) / DET
+      AINV(3,1) =  (A(2,1)*A(3,2)-A(2,2)*A(3,1)) / DET
+      AINV(3,2) = -(A(1,1)*A(3,2)-A(1,2)*A(3,1)) / DET
+      AINV(3,3) =  (A(1,1)*A(2,2)-A(1,2)*A(2,1)) / DET
+      END SUBROUTINE MAT_INV3
+
+! ---------------------------------------------------------------------------
+      SUBROUTINE NEOHOOK_S2PK(CMAT, MU, DPEN, S2PK, TRC)
+      IMPLICIT NONE
+      DOUBLE PRECISION CMAT(3,3), S2PK(3,3)
+      DOUBLE PRECISION MU, DPEN, TRC
+      DOUBLE PRECISION CMATINV(3,3), I3(3,3), J
+      DOUBLE PRECISION MAT_DET3
+      EXTERNAL MAT_INV3
+
+      CALL MAT_IDENTITY(I3)
+      J = DSQRT(MAT_DET3(CMAT))
+      TRC = CMAT(1,1) + CMAT(2,2) + CMAT(3,3)
+      CALL MAT_INV3(CMAT, CMATINV)
+      S2PK = MU * I3 + (2.0D0/DPEN) * (J - 1.0D0) * J * CMATINV
+      END SUBROUTINE NEOHOOK_S2PK
+
+! ---------------------------------------------------------------------------
+      DOUBLE PRECISION FUNCTION MAT_DET3(A)
+      IMPLICIT NONE
+      DOUBLE PRECISION A(3,3)
+      MAT_DET3 = A(1,1)*(A(2,2)*A(3,3)-A(2,3)*A(3,2))
+     &          - A(1,2)*(A(2,1)*A(3,3)-A(2,3)*A(3,1))
+     &          + A(1,3)*(A(2,1)*A(3,2)-A(2,2)*A(3,1))
+      END FUNCTION MAT_DET3
+
+! ---------------------------------------------------------------------------
+      SUBROUTINE ZERO_TANGENT(DDSDDE, NTENS)
+      IMPLICIT NONE
+      INTEGER NTENS, I, J
+      DOUBLE PRECISION DDSDDE(NTENS,NTENS)
+      DO I=1,NTENS
+        DO J=1,NTENS
+          DDSDDE(I,J) = 0.0D0
+        END DO
+      END DO
+      END SUBROUTINE ZERO_TANGENT
+
+! ---------------------------------------------------------------------------
+      SUBROUTINE ISOTROPIC_TANGENT(DDSDDE, LAMBDA, MU)
+      IMPLICIT NONE
+      DOUBLE PRECISION DDSDDE(6,6), LAMBDA, MU
+
+!     3D isotropic elasticity in Voigt order
+      DDSDDE(1,1) = LAMBDA + 2.0D0*MU
+      DDSDDE(2,2) = LAMBDA + 2.0D0*MU
+      DDSDDE(3,3) = LAMBDA + 2.0D0*MU
+      DDSDDE(1,2) = LAMBDA
+      DDSDDE(1,3) = LAMBDA
+      DDSDDE(2,1) = LAMBDA
+      DDSDDE(2,3) = LAMBDA
+      DDSDDE(3,1) = LAMBDA
+      DDSDDE(3,2) = LAMBDA
+      DDSDDE(4,4) = MU
+      DDSDDE(5,5) = MU
+      DDSDDE(6,6) = MU
+      END SUBROUTINE ISOTROPIC_TANGENT