# PEDA-NOCV for Spin Unrestricted Calculations¶

This tutorial will teach you how to:

• perform an energy decomposition analysis (PEDA) combined with the natural orbitals for chemical valency (NOCV) method (pEDA-NOCV) for extended and/or molecular systems with the BAND-GUI, where the bond is described by open-shell, spin unrestricted fragments
• where to look for the results

Start ADFinput in a clean directory. (according to Step 1 of the Getting Started with BAND)

Switch to the BAND main panel
Chose Periodicity: None.

## Step 2: Set up the system - Ethane¶

An easy and non-timeconsuming test system is the C-C bond analysis in an ethane molecule. Here, two methyl radicals with opposite spin-polarization do interact to form the shared electron C-C bond. Hence, the fragments and the PEDA-NOCV calculation have to be carried out as unrestricted DFT calculations.

You can copy-paste the following geometry information into the GUI directly.

C          1.54081631       0.00000000       0.00000000
H          1.90061013       0.71448558       0.72714386
H          1.90061011      -0.98692880       0.25534173
H          1.90084793       0.27238885      -0.98228957
C          0.00000000       0.00000000       0.00000000
H         -0.35953041      -0.27213306       0.98136715
H         -0.36080772       0.98640196      -0.25411759
H         -0.36080775      -0.71466441      -0.72582318


## Step 3: Running the PEDA-NOCV calculation¶

### Step 3a: Setting up the fragments¶

To run the PEDA and PEDA-NOCV you have to define fragments. Therefore switch to Regions menu.

Panel bar Model → Regions
• Select one methyl fragment and add a new region by clicking on the ‘+’ button (or Ctrl+G).
• Then select the other methyl fragment and add a new region by clicking on the ‘+’ button (or Ctrl+G).
• You may want to rename “Region_1” to “H3C_A” and “Region_2” to “H3C_B”. (optional)

### Step 3b: Details for the calculation¶

Panel bar Main

and change the calculation setup (XC functional, basis set, frozen core, numerical quality and unrestricted calculation) according to the following picture.

### Step 3c: Enabling the PEDA-NOCV¶

Panel bar MultiLevel → Fragments
• Check the “Use fragments” box. This will trigger the PEDA.
• Define the spin polarization of the fragments. One shall be +1 (excess of 1 electron with spin up) and the other -1 (excess of 1 electron with spin down).

Furthermore, enable the PEDA-NOCV calculation as follows:

Panel bar Properties → PEDA-NOCV
Check the Perform PEDA-NOCV analysis box
Set Use NOCVs with eV larger than: to 0.001

### Step 3d: Save and run the calculation¶

Now you can save and run the calculation.

File → Save, give it a name and press Save.
File → Run

## Step 4: Checking the results¶

After the calculations of the fragments and the PEDA-NOCV finished you can look for the PEDA results. Therefore, open the “Output” using the SCM dropdown menu.

SCM → Output

You can jump to the ‘PEDA Energy Terms’ via the corresponding button in the ‘Properties’ dropdown menu.

Properties → PEDA Energy Terms

Reference results:

In addition to these energy terms the summed preparation energies of the fragments and the (negative) bond dissociation energy are usually given. Therefore you have to calculate the energy difference between the electronically and structurally relaxed fragments (which can be accessed by a geometry optimization of the separated fragments) and the promoted fragments (which are already calculated and used for the PEDA). Adding this energy differences, which are equal to the preparation energy, to the interaction energy will give you the negative bond dissociation energy.

You can jump to the ‘PEDA-NOCV Energy Terms’ via the corresponding button in the ‘Properties’ dropdown menu.

Properties → PEDA-NOCV Energy Terms

Reference results:

## Step 5: Plotting NOCV orbitals and deformation densities¶

You can visualize the charge NOCV deformation densities which describe the charge flow between the fragments. Therefore, open the “View” using the SCM dropdown menu.

SCM → View
Fields → Grid → Medium

Depending on your preferences (w.r.t. showing atoms of neighboring cells) you will end up with the following representation of ADFview.

(new Image compare to other PEDANOCV tutorial)