Alternatively, or in addition, researchers can use an existing code or product.. As examples, CalcUS, WebMO, ChemCompute 39 and MolCalc 40 are all web-based, calculation submission platforms. They provide tools to draw and/or upload molecular structures, chose calculation options, and submit a calculation to a number of backend computational chemistry programs. They also offer common analysis options, such as the parsing of molecular orbital energies, the plotting of molecular orbitals and the visualization of vibrational modes, although the exact feature set differs between each program. They all interface to a queuing system, either coming bundled with an internal queuing manager (CalcUS) or interfacing to external programs, such as SLURM 41 (WebMO). CalcUS and WebMO are targeted towards academic research, while ChemCompute and MolCalc are designed for undergraduate teaching. CalcUS and MolCalc are free and open-source software, while ChemCompute is offered as a free-to-use service only, and WebMO is commercial, although it does offer a free basic version with a reduced feature set. Winmostar 42 is a similar calculation submission platform, but is designed to be run on the user's desktop rather than in a web-browser, and is also commercial. Maestro, 43 Spartan, 44 TMoleX 45 and GaussView 46 are graphical user-interfaces (GUIs) to the computational engines Jaguar, 47 Q-Chem, Turbomole, and Gaussian, respectively, and each is specific to its own engine. Maestro is typically distributed as part of the commercial Schrodinger platform, along with Jaguar, which, according to the authors, 43 is targeted primarily towards researchers in drug discovery. Spartan is sold as a standalone product, with Q-Chem bundled inside of it, while Gaussview is sold as an addon for Gaussian, and TMoleX is included in the purchase of Turbomole. All four offer common analysis features, such as parsing of molecular orbital energies, vibrational frequencies, and excited states, along with more advanced functionality such as the plotting of molecular orbital densities. They differ somewhat in their support for submitting calculations. Both Gaussview and TMoleX can either write input files to the hard drive, or call the underlying engine directly, if installed on the same machine. TMoleX additionally supports calling remote installations of Turbomole. Spartan and Maestro, on the other hand, are designed to perform the calculation internally, without calling an external engine, although both can additionally interface to remote installations. Molden, Gabedit 52 and Avogadro 53 are more general tools and interface to a number of backend programs. They offer the usual analysis functions such as molecular orbital plotting, as well as input file creation tools, but do not manage the running of the calculation itself.