# General hints - frequently asked questions

Molpro users should have a basic knowledge of quantum chemistry, from text books as recommended below.

Also, a basic knowledge of Linux / macOS is required.

As an introduction, studying the examples in this manual is recommended. Also, the Molpro forum at https://groups.google.com/g/molpro-user provides a lot of information. We request to post questions concerning Molpro to this forum.

Text books:

• A. Szabo and N.S. Ostlund: Modern Quantum Chemistry (McGraw-Hill, New York, 1989)
• T. Helgaker, P. Jørgensen, J. Olsen: Molecular Electronic-Structure Theory (Wiley, Chichester, 2000)
• F. Jensen: Introduction to Computational Chemistry (Wiley, Chichester, 2017)
• Christopher J. Cramer: Essentials of Computational Chemistry: Theories and Models (Wiley, Chichester, 2004)
• W.J. Hehre, L. Radom, P.v.R. Schleyer, and J.A. Pople: Ab initio molecular orbital theory (Wiley, New York, 1986)
• W. Kutzelnigg: Einführung in die Theoretische Chemie (Wiley-VCH, Weinheim, 2001)
• Computational parameters such as tolerances, thresholds have been carefully calibrated. Usually they do not need to be changed.
• gprint,basis,orbitals may be inserted so that these data are available in the output file; it however makes the output file larger.
• Molpro uses a default memory size (see here) which should be sufficient for small systems. When requesting more memory, it is recommended to first get some experience about the memory necessary, and then request a reasonable amount of memory, and not astronomic memory for small systems.
• The units have to be correct (Angstrom versus bohr). By default, xyz input is in Angstrom, while z-matrix coordinates are in atomic units.
• A geometry which strongly deviates from the experimental geometry may cause convergence problems. This may be due to unrealistic bond lengths or angles. E.g. breaking a bond may require to change the wave function card or method (remember the text book example of H2 with a large bond length, RHF versus UHF).
• If any kind of problems in a calculation occur, it is recommended to visualise the geometry and check it for correctness.
• The basis set has to be chosen with respect to the method, and with respect to the property to be computed.
• Negatively charged atoms need diffuse basis functions. A typical example is a molecule containing oxygen, where oxygen usually becomes negatively charged.
• Symmetry should be exploited if possible. The calculations are faster, easier to converge, and easier to analyse if symmetry is used.
• It should be checked if the symmetry and spin state is correct. The program attempts to find the ground state configuration from the Aufbau principle, but this may fail and then the wave function symmetry and spin need to be specified using a WF (wave function) directive.
• After convergence, especially of difficult cases, it is recommended to check orbitals and populations, possibly visualise orbitals.
• After converging a difficult system, it may occasionally be necessary to reorder the orbitals (“rotate” command) before performing a subsequent correlation calculation.

Before increasing the number of iterations to very high values (such as “maxit,500”) far beyond the default:

• check the geometry
• check the initial orbitals (especially in case of MRCI)
• check the choice of the active space

• The active space (occ, frozen, closed) has to be chosen carefully. (Nearly) degenerate states and low lying excited states should be included, based on e.g. SCF eigenvalues.
• AVAS may help to construct suitable starting orbitals.
• The active space has to be reasonably chosen (occ, core, closed); e.g. typically after an MCSCF calculation, an MRCI calculation is followed with the same active space.
• When excited states are computed, then the preceding MCSCF should be state averaged, and include the excited states so that the orbitals from which the MRCI starts are reasonable. E.g.  multi;occ,14,4,9,2;frozen,12,1,6,0;wf,47,1,1;state,3 may be followed by mrci;occ,14,4,9,2;core,12,1,6,0;wf,47,1,1;state,3;
• MCSCF and MRCI are not black-box like, and e.g. a visualisation of the MCSCF orbitals will help to set up the active space.