In order to provide theoretical and computational chemists with an efficient, user-friendly and unique atomistic computing software packages for quantum chemistry and artificial intelligence calculations of atomistic systems, Xiamen University has recently released the Xiamen Atomistic Computing Suite (XACS). XACS consists of three core software packages:
• XMVB – A Valence Bond Theory Based Quantum Chemistry Program
• XEDA – A General and Multipurpose Energy Decomposition Analysis Program
• MLatom – A Package for Atomistic Simulations with Machine Learning

We will cover the use of the above three packages in details in three articles and will focus on the XMVB software in this article.
As one of the modern chemical bonding theories, valence bond theory can provide intuitive chemical pictures for molecular structural properties and chemical reaction mechanisms. XMVB is a quantum chemistry program based on classical valence bond theory for ab initio electronic-structure calculations by using non-orthogonal orbitals and it has implemented all the mainstream VB methods and functions. XMVB can be used for the nature of chemical bonding theory and has become the accurate multi-configuration quantum chemistry program. Besides, XMVB has been taken as the only VB software in several international VB theory seminars. XMVB is the most popular and influential software for ab initio valence bond calculations in the world. There has been several international monographs or toolkits that include chapters on how to use XMVB.

There are two ways to perform XMVB calculations: one is from online cloud computing and the other is to download and install the software on your local server. This article focuses on XMVB cloud computing.
1. How to use
With XACS cloud computing, users do not need to download or install software packages. They can simply visit the XACS official website and then register a free account to perform XACS calculations online, track the progress of the calculations and obtain the results. Once you have logged into your account, you can visit the XMVB cloud computing page to perform XMVB calculations. To help new users get started with XMVB, we have prepared a number of example calculations for your reference.
Before VB calculation, we need to get the molecular orbital information of the system and then determine the number of active electrons and basis functions of the active orbitals. Therefore, for new systems under study, we need to first perform Hartree-Fock calculation, which can be done by Preliminary calculation ((toggle this option in online interface). If the molecular orbital information for the system is known, we can skip the Preliminary calculation step and prepare input files for the VB calculation directly. Then submit the jobs to start the VB calculation.

2. Example
Take the benzene molecule as an example, which is available on the XACS website (https://xacs.xmu.edu.cn/records?example_id=113&editable=0&attribution_program=XMVB). Here is the input file for the preliminary calculation.

Visit the XMVB cloud computing page and set the Preliminary calculation option to be yes. Then copy the above input file into the text box (shown below). Finally click Submit to start the Hartree-Fock calculation.

To study the bonding properties of the 6 π-electrons of the benzene molecule, we will do VBSCF calculation on the active space of (6e,6o) with 6-31G basis set. The input file for the VB calculation can be prepared according to the output file from the Preliminary calculation as follows.
In the input file, the "str=cov" in $ctrl part indicates that only covalent structures are considered, and the $geo section gives the molecular coordinates of benzene. The $orb section gives information about the basis functions of each VB orbital, where the first 18 orbitals represent double-occupied inactive sigma-orbitals while the last 6 represent active pi-orbitals localized on C atoms.

Once you have the input file, go to the XMVB Cloud Computing page and set Preliminary calculation to be no, then copy the content of the input file into the text box (as shown above) and click Submit to start the calculation. During the calculation, click “my computing” at the bottom of the page to track the progress of the job in time. Once the calculation is complete, you can view the results via XGUI software in the suite. After opening XGUI, create a new project named c6h6 at first, and then double-click on the “xmo” in the project to import the .xmo output file in your local machine, and click on "Structures" to view the five covalent structures involved in the VB calculation, including 2 Kekulé structures and 3 Dewar structures. The results of these structures are shown below.

The coefficients and weights of the 2 Kekulé and 3 Dewar structures are the same, respectively, which is consistent with the symmetry of the VB structure.
Welcome to visit the XACS website for more help. If you have any questions, please contact us via xmvb@xmu.edu.cn.
XMVB manual: http://xacs.xmu.edu.cn/softwares/xmvb-3.0-manual.pdf
XMVB tutorial: http://xacs.xmu.edu.cn/softwares/XMVB_tutorial_2022.pdf
XMVB examples: https://xacs.xmu.edu.cn/examples?attribution_program=XMVB