The TREX project is in the field of high-accuracy quantum chemical and materials simulations with a special focus on Quantum Monte Carlo (QMC) approaches to the solution of the quantum many-body problem at the heart of atomistic physics, chemistry, and materials science. Importantly, due to their inherent parallelizability and high computational cost, QMC approaches, and thus TREX is uniquely positioned to fully exploit the massive parallelism of the upcoming exascale supercomputer architectures. The software developed in this project for improving code inter-operability and interfacing will be referenced in the following as TREXIO. The library was released in the previous D2.2 deliverable and is being continuously updated to facilitate its use and maximize its portability. This document will be the basis for the next D2.5 and the final D2.6 deliverable. Let us remind the main idea of TREXIO that can be useful within and outside TREX CoEs. In a brute force approach, each developer, in order to interface its own code (e.g. TurboRVB) with any other code (see Fig.1, outer and inner shells), is forced to write a different interface for each of them, and this is a clearly inefficient waste of human resources. Instead with the TREXIO library, by means of highly developed and standardized input and output paradigms, only one interface is necessary: the one with the TREXIO library. This clearly represents an enormous simplification of the effort. Moreover, once all planned interfaces (green balls in Fig.1) will be available the visibility and the scientific appeal of the TREX codes, will be clearly enhanced. In this report, we will mainly focus on the pre-release of an integrated platform with inter-operable Open Source flagship codes, including the I/O. This library is important for the gradual integration of the QMCkl library (its status has been recently reported in the D3.2 deliverable) in the TREX flagship codes. All the technical aspects related to this important development will be dealt with in the next D3 reports.