# Interoperability between Building Information Modelling (BIM) and Building Energy Model (BEM)
## 1 Introduction
Due to this, at present, there is a significant interest in simplifying the creation of a BEM from the BIM authoring tool, by sharing and exporting the data from the architectural model [23; 24]. However, BIM-BEM interoperability is one of the existing digital gaps at the design stage. Although BIM is considered a multidisciplinary tool, interoperability issues still prevent many BIM applications in relevant industries [25]. The lack of compatibility between the BIM model and the energy simulation applications, the repetitive manual operations required to create a BEM, and the non-standardised and subjective process [26] usually result in data loss and misinterpretation, especially when this was been taken into account in the first phase of the design step [27; 28]. [PERSON] et al. also concluded that the main problem in transferring data between BIM and BEM is the information loss, which can lead to a rework consisting of re-entering the BIM stored information into the energy model [29]. Furthermore, [PERSON] and [PERSON] were in accordance with [PERSON] et al., defending that BIM contributes to the easy handling of data, which can leads to automation in building energy modelling and simulation, as well as providing up-to-date energy models with real-time information. They also emphasised the need for clear standards and solutions for the BIM to BEM interoperability process [30]. The BIM-BEM process consists of three parts--BIM tool, model schema exchange format, and BEM software--interoperability issues may appear in any or all of these; not only in the energy simulation application. This means the problems that are encountered are not exclusively due to the limited ability of BEM tools to translate input data from BIM [31]. In order to transfer the BIM information to a building energy analysis software, many file formats exist, such as hypertext markup language (HTML), extensible hypertext markup language (XHTML), building construction extensible markup language (bcXML), industry foundation classes extensible markup language (IFCXML), industry foundation classes (IFC), and green building extensible markup language (gbXML), the last two being the main open BIM standards [32]. Open standards enable and facilitate the integration and collaboration of different professionals from all over the world, by providing a common basis around which research, development, and deployment activities can be organised, as has been done in the IBPSA Project 1 [33]. These open standards are commonly used in the BIM-BEM procedure, and there is an ongoing discussion about which standard is the best for the data exchange process. On the one hand, IFC reproduces a complete building project and, therefore, generates a complex data schema and larger size data files. On the other hand, gbXML provides a more flexible and direct approach to energy analysis [34]. Both of them transfer the material properties, data for HVAC systems, and thermal zones; however, only gbXML provides location data [30]. However, [PERSON] et al. drew different conclusions about gbXML and IFC data transfer. They invented a classification process called BIM-based-BEM, which divides the information transfer from BIM into six categories: Geometry (step 1), material (step 2), space type (step 3), thermal zone (step 4), space load (step 5), and HVAC (step 6). After analysing the information transfer between distinctive softwares, they verified that the IFC format is step 1 and gbXML is step 3. In addition, they concluded that the information transfer is not user-friendly, as there are many steps between BIM and BEM [35; 36]. In summary, BIM-BEM interoperability has not yet become as widespread as expected, due to the information loss, lack of a standard, software not being as interoperable as claimed by their suppliers, and the data standards not having been clearly determined [37].