EDA is used to partition the total energy of a molecular system into individual energy contributions from different fragments or interactions. While Gaussian 09 (G09) is a widely used quantum chemistry software package, it does not have built-in functionality specifically designed for Energy Decomposition Analysis. However, you can perform EDA using G09 by employing an external software or post-processing tool. Here's a general outline of the steps involved:

1. Perform the quantum chemical calculation: Run a quantum chemical calculation using G09 to obtain the total energy of the molecular system of interest. Ensure that you choose an appropriate level of theory and basis set for your study.

2. Extract fragment energies: Use G09's output files to extract the energies of individual fragments or components of interest. This typically involves identifying the relevant sections in the output file and collecting the corresponding energy terms.

3. Perform a reference calculation: Perform additional quantum chemical calculations to obtain reference energies for isolated fragments or reference states. These reference calculations should be performed using the same level of theory and basis set as the main calculation.

4. Perform the energy decomposition: Use an external software or post-processing tool to perform the energy decomposition analysis. There are several popular tools available for this purpose, such as:

- Morphy: It is a software package specifically designed for Energy Decomposition Analysis. It can perform various EDA methods, including SAPT (Symmetry-Adapted Perturbation Theory) and SAPT(DFT) (Density Functional Theory-based SAPT). Morphy has a user-friendly interface and can be used to analyze G09 output files.

- ADF (Amsterdam Density Functional): ADF is a comprehensive quantum chemistry software package that includes the capability to perform EDA. It offers the ADF-GUI interface, where you can load G09 output files and perform energy decomposition using methods like SAPT and DFT-D.

- Multiwfn: Multiwfn is a versatile post-processing tool for wavefunction analysis. It supports energy decomposition analysis using several methods, including SAPT and NBO (Natural Bond Orbital) analysis. You can use Multiwfn to analyze G09 output files and extract energy contributions.

5. Interpret the results: Analyze the obtained energy decomposition results to understand the contributions from different interactions or components in your system. This analysis can provide insights into the nature and strength of various intermolecular or intramolecular interactions.

Please review the documentation and user guides of the external software or post-processing tools you choose to perform the energy decomposition analysis. These tools may have specific input file formats or requirements that you need to follow.

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