Q-Chem

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User Manual

Q-Chem v2.0 Manual (On vergil)

Q-Chem v2.1 Manual (On vergil)

Bugs


Program Quirks

DFT

Large DFT Optimizations

QChem will sometimes fail unexpectedly during large optimizations when it reaches the wavefunction analysis (which is on by default). To turn off the wavefunction analysis include the keyword:

 IANLTY       0

If the optimization fails at a later point after including this keyword, try increasing the memory:

 MEM_TOTAL             3000 
 MEM_STATIC            256

This would allocate 3 GB total for the program and increase the amount of static memory to 256 MB. This amount of static memory seems to work well; do not make the static memory too large of a portion of the total memory or the program will have additional problems. This amount of memory will run a B3LYP optimization with at least 1000 basis functions, probably significantly more.

Hohenstein 00:01, 22 November 2008 (EST)

Coupled Cluster

Frequencies

In order to compute vibrational frequencies with any coupled cluster method in QChem numerical derivatives are used either from gradients (CCSD) or single point energies (notably CCSD(T)). If your molecule has symmetry, the coupled cluster code will get confused when it turns symmetry off to do the displacements. You must include the keyword:

 CC_SYMMETRY     FALSE

This turns off the use of point group symmetry in the coupled cluster code. Supposedly symmetry is turned off by default for finite difference calculations, but this does not appear the case.

Hohenstein 23:47, 21 November 2008 (EST)

Basis Sets

Ghost Atoms and Mixed Basis Sets

In the event that you want to basis functions at locations that do not correspond to atom centers or if you want to use different basis sets on different atom centers, you must use a mixed basis set. This applies to counterpoise corrections and the use of bond functions. QChem requires that you specify the basis set and the coefficients (by atom type) to use on each center. This has the form:

  $molecule
    0 1
    O    1.68668 -0.00318  0.000000
    H    1.09686  0.01288 -0.741096
    H    1.09686  0.01288  0.741096
    Gh  -1.45451   0.01190  0.000000
    Gh  -2.02544  -0.04298 -0.754494
    Gh  -2.02544  -0.04298  0.754494
  $end
  $rem
    EXCHANGE  HF
    BASIS     MIXED
  $end
  $basis
    O 1
    aug-cc-pVDZ
    ****
    H 2
    cc-pVDZ
    ****
    H 3
    cc-pVDZ
    ****
    O 4
    aug-cc-pVDZ
    ****
    H 5
    cc-pVDZ
    ****
    H 6
    cc-pVDZ
    ****
  $end

The "Gh" atoms correspond to dummy centers. In this case, these are getting basis functions for aug-cc-pVDZ oxygen and cc-pVDZ hydrogen.

Hohenstein 20:53, 21 November 2008 (EST)

Cartesian and Spherical Harmonic Basis Functions

QChem will try to use the correct angular form for its predefined basis sets. QChem defaults to pure angular forms for Dunning's basis sets. QChem will default to cartesian d-type functions for the 3-21G and 6-31G series. It defaults to pure d- and f-type functions for the 6-311G series.

Mixed basis sets or custom basis sets default to cartesian h-functions and pure g-, f-, and d-type functions. To change this, use the following keyword:

 PURECART    hgfd

Where 1 denotes use of pure functions and 2 denotes use of cartesian functions. In this notation, QChem defaults to:

 PURECART    2111

Hohenstein 21:03, 21 November 2008 (EST)

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