message: zero accelerations are good for stability
diff: @@ -1410,6 +1410,7 @@ void TACSBDFIntegrator::evalFunctions( TacsScalar *fvals ){
  fvals[n] = funcs[n]->getFunctionValue();
  }
  }
  +
  }
  /*
  @@ -1933,6 +1934,9 @@ int TACSDIRKIntegrator::iterate( int k, TACSBVec *forces ){
  qddotS[offset]->copyValues(qddotS[offset-1]);
  }
  + //
  + qddotS[offset]->zeroEntries();
  +
  // Compute approximations for qS
  qS[offset]->copyValues(q[k-1]);
  qS[offset]->axpy(h*c[stage], qdot[k-1]);
  