# Generation of Cube Files — `cubeprop()`

Code author: Robert M. Parrish, Francesco A. Evangelista and Peter Kraus

Section author: Francesco A. Evangelista and Peter Kraus

`psi4.``cubeprop`(wfn)

  Evaluate properties on a grid and generate cube files.

  New in version 0.5: wfn parameter passed explicitly

  Returns: | None
  Parameters: | wfn (`Wavefunction`) – set of molecule, basis, orbitals from which to generate cube files
  Examples: |

      >>> # [1] Cube files for all orbitals
      >>> E, wfn = energy('b3lyp', return_wfn=True)
      >>> cubeprop(wfn)

      >>> # [2] Cube files for density (alpha, beta, total, spin) and four orbitals
      >>> #     (two alpha, two beta)
      >>> set cubeprop_tasks ['orbitals', 'density']
      >>> set cubeprop_orbitals [5, 6, -5, -6]
      >>> E, wfn = energy('scf', return_wfn=True)
      >>> cubeprop(wfn)

## Introduction

PSI4 has the ability to export cube files that store information about basis functions, molecular orbitals, the electron density, and the electrostatic potential (ESP). Cube files store the value of a scalar quantity on a regular Cartesian grid, and can be visualized with several visualization programs, some of which are free, like VMD (http://www.ks.uiuc.edu/Research/vmd/).

An example utilization of the code is:

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    molecule h2o {
    0 1
    O
    H 1 1.0
    H 1 1.0 2 104.5
    }

    set basis cc-pvdz
    set scf_type df
    set freeze_core True
    set cubeprop_tasks ['orbitals']
    set cubeprop_orbitals [5,6,-5,-6]

    E, wfn = energy('scf', return_wfn=True)
    cubeprop(wfn)

In this example, the `cubeprop()` call after the `energy('scf')` command executes the cubeprop code. The array CUBEPROP_TASKS specifies which tasks should be executed. In this case the task `'orbitals'` generates cube files for orbitals. The CUBEPROP_ORBITALS option specifies that cube files should be generated only for alpha orbitals 5 (HOMO) and 6 (LUMO) and beta orbitals 5 (indicated as -5) and 6. If the option CUBEPROP_ORBITALS is not provided, then cube files are generated for all orbitals. After running, the above input will generate four files: `Psi_a_5_1-B1.cube`, `Psi_a_6_4-A1.cube`, `Psi_a_5_1-B1.cube`, and `Psi_a_6_4-A1.cube`. The subscript `a` in `Psi_a_5_1-B1.cube` indicates an alpha orbital. The first number (`5`) is the index of the orbital while `1-B1` indicates that this is the first orbital that belongs to the B1 irrep. The file `Psi_a_5_1-B1.cube` begins with two comment lines:

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    Psi4 Gaussian Cube File.
    Property: Psi_a_5_1-B1. Isocontour range for 85% of the density: (0.0787495,-0.0787495)

The second line reports the isocontour values that capture 85% of the probability density using the least amount of grid points. This quantity is determined for orbitals and densities. The fraction of the density captured by the isocontour values is by default 0.85, but can be changed via the option CUBEPROP_ISOCONTOUR_THRESHOLD.

Note

If your cube plots are too coarse, try to decrease the grid spacing via the option CUBIC_GRID_SPACING. If the edges of your plot are cut then increase the size of the grid via the option CUBIC_GRID_OVERAGE.

## Cubeprop Tasks

The cubeprop utility can be provided a list of tasks to perform. Tasks are specified by the CUBEPROP_TASKS option, which is a list of strings that identify the tasks. Several tasks are available. These include:

ORBITALS [Default if CUBEPROP_TASKS is not specified]
  Produces cube representations of the molecular orbitals \(\psi_q(\mathbf{r})\). Orbitals are sorted according to increasing orbital energy ignoring symmetry.
FRONTIER_ORBITALS
  Produces cube representations of the frontier molecular orbitals. For closed shell species, the highest occupied (HOMO) and the lowest unoccupied (LUMO) alpha orbitals (ie. \(\psi_{\alpha}(\mathbf{r})\)) are printed, while for open shell species a total of \((4 + M_s)\) orbitals are printed (\(\alpha\) and \(\beta\) spin for both lowest virtual (LVMO) and highest doubly occupied orbitals (DOMO), along with all \(\alpha\) singly occupied (SOMO) orbitals).
DENSITY
  This task can be used to obtain the alpha and beta electron densities, \(\rho_\alpha(\mathbf{r})\) and \(\rho_\beta(\mathbf{r})\), together with the total density \(\rho(\mathbf{r}) = \rho_\alpha(\mathbf{r}) + \rho_\beta(\mathbf{r})\), and the spin density \(\rho(\mathbf{r}) = \rho_\alpha(\mathbf{r}) - \rho_\beta(\mathbf{r})\).
BASIS_FUNCTIONS
  This task is useful to produce cube files of the atomic orbital basis functions \(\chi_\mu(\mathbf{r})\).
ESP
  Calculates the total (nuclear + electronic) electrostatic potential \(V(\mathbf{r})\).
DUAL_DESCRIPTOR
  Calculates the dual descriptor from frontier orbitals: \(f^2(\mathbf{r})=\rho_{\mathrm{LUMO}}(\mathbf{r})-\rho_{\mathrm{HOMO}}(\mathbf{r})\). The dual descriptor is a good measure of nucleophilicity and electrophilicity, containing information essentially equivalent to both Fukui functions combined. More details on the dual descriptor itself can be found in [Morell:2005:205], while the current implementation is described in [Martinez-Araya:2015:451]. This feature is currently only supported for closed shell systems.

Note

The `ESP` task requires the user to specify a density-fitting basis via the DF_BASIS_SCF keyword.

Warning

It is important to specify the CUBEPROP_ORBITALS option when dealing with large molecules to avoid running out of disk space. For example, using the default grid spacing of 0.2 Ångström, the size of a single cube file for a molecule like water is of the order of 1.4 MB. For a molecule with 200 basis functions, the cube files for all the orbitals occupy more than half a GB.

## Keywords

### CUBEPROP_TASKS

> Properties to compute. Valid tasks include: `DENSITY` - Da, Db, Dt, Ds; `ESP` - Dt, ESP; `ORBITALS` - Psi_a_N, Psi_b_N; `BASIS_FUNCTIONS` - Phi_N; `LOL` - LOLa, LOLb; `ELF` - ELFa, ELFb; `FRONTIER_ORBITALS` - Psi_a_N_HOMO + Psi_a_N_LUMO; `DUAL_DESCRIPTOR` - DUAL_N_HOMO-M_LUMO.

> - Type: array
> - Default: No Default

### CUBEPROP_FILEPATH

> Directory to which to write cube files. Default is the input file directory.

> - Type: string
> - Default: No Default

### CUBEPROP_ORBITALS

> List of orbital indices for which cube files are generated (1-based, \(+\) for alpha, \(-\) for beta). All orbitals computed if empty.

> - Type: array
> - Default: No Default

### CUBEPROP_BASIS_FUNCTIONS

> List of basis function indices for which cube files are generated (1-based). All basis functions computed if empty.

> - Type: array
> - Default: No Default

### CUBIC_GRID_SPACING

> CubicScalarGrid grid spacing in bohr [D_X, D_Y, D_Z]. Defaults to 0.2 bohr each.

> - Type: array
> - Default: No Default

### CUBIC_GRID_OVERAGE

> CubicScalarGrid spatial extent in bohr [O_X, O_Y, O_Z]. Defaults to 4.0 bohr each.

> - Type: array
> - Default: No Default

# Orbital Visualization with VMD

Included in PSI4 is functionality to automatically render specified surfaces, including molecular orbitals, densities, and basis functions, using VMD. The vmd_cube.py script takes the .cube files generated in a calculation and generates images aligned with user-input specifications. The script is located in psi4/psi4/share/psi4/scripts/vmd_cube.py.

## Script Prerequisites

1. VMD must be installed, and it can be downloaded for free at (http://www.ks.uiuc.edu/Research/vmd/). Additionally, the script needs to know where to find the VMD executable, and this is defined as `VMDPATH`. VMDPATH must be defined as an environment variable.
2. To generate images with multiple surfaces, ImageMagick must also be installed. ImageMagick is a free program which can be installed using homebrew/pip or from http://www.imagemagick.org/script/binary-releases.php .
3. With ImageMagick installed, an environment variable called `MONTAGE` needs to be created which points to the montage executable. This executable can be found in the /bin/ sub-directory wherever ImageMagick was installed.

## Running the Script

1. Run a PSI4 calculation, generating .cube files as detailed in the above documentation.

2. Add vmd_cube.py to your path. Call vmd_cube.py and pass the directory containing the .cube files as an argument:

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       python vmd_cube.py /path/to/cube/files/

   By default the script will search for cube files in the directory in which it is called. In addition to individual image files for each orbital, vmd_cube will also generate an image containing all surfaces in an array (very useful for hand-picking orbital spaces)

3. As an example, take the cube files generated from the water calculation from the above input file. Using vmd_cube the alpha molecular orbitals can be rendered and output in one image with:

> python vmd_cube.py /path/to/cube/files/ –opacity=0.5 –rx=90 –ry=60

The desired image in this case is called “AlphaMOs.tga”, and looks like this:

## Script Options

The help for `vmd_cube.py` can be obtained from the console:

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    >>> ./vmd_cube.py --help
    usage: vmd_cube.py [-h] [--color1 [<integer>]] [--color2 [<integer>]]
                       [--iso [<isovalue>]] [--rx [<angle>]] [--ry [<angle>]]
                       [--rz [<angle>]] [--tx [<length>]] [--ty [<length>]]
                       [--tz [<length>]] [--opacity [<opacity>]]
                       [--scale [<factor>]] [--no-montage] [--no-labels]
                       [--imagesize [<integer>]] [--imagew [<integer>]]
                       [--imageh [<integer>]] [--fontsize [<integer>]]
                       [--interactive] [--gzip] [--national_scheme]
                       [--silver_scheme] [--bright_scheme] [--electron_scheme]
                       [<cubefile dir>]

    vmd_cube is a script to render cube files with vmd. To generate cube files
    with Psi4 add the command cubeprop() at the end of your input file.

    positional arguments:
      <cubefile dir>        The directory containing the cube files.

    optional arguments:
      -h, --help            show this help message and exit
      --color1 [<integer>]  the color ID of surface 1 (integer, default = 3)
      --color2 [<integer>]  the color ID of surface 2 (integer, default = 23)
      --iso [<isovalue>]    the isosurface value (float, default = 0.05)
      --rx [<angle>]        the x-axis rotation angle (float, default = 30.0)
      --ry [<angle>]        the y-axis rotation angle (float, default = 40.0)
      --rz [<angle>]        the z-axis rotation angle (float, default = 15.0)
      --tx [<length>]       the x-axis translation (float, default = 0.0)
      --ty [<length>]       the y-axis translation (float, default = 0.0)
      --tz [<length>]       the z-axis translation (float, default = 0.0)
      --opacity [<opacity>]
                            opacity of the isosurface (float, default = 1.0)
      --scale [<factor>]    the scaling factor (float, default = 1.0)
      --no-montage          call montage to combine images. (string, default =
                            false)
      --no-labels           do not add labels to images. (string, default = false)
      --imagesize [<integer>]
                            the size of each image (integer, default = 250)
      --imagew [<integer>]  the width of images (integer, default = 250)
      --imageh [<integer>]  the height of images (integer, default = 250)
      --fontsize [<integer>]
                            the font size (integer, default = 20)
      --interactive         run in interactive mode (default = false)
      --gzip                gzip cube files (default = false)
      --national_scheme     use a red/blue color scheme. (string, default = false)
      --silver_scheme       use a gray/white color scheme. (string, default =
                            false)
      --bright_scheme       use a soft yellow/blue color scheme. (string, default
                            = false)
      --electron_scheme     use a purple/green color scheme. (string, default =
                            false)