### Welcome to the OpenSolver on-line help.

We hope that most of your questions will be answered on the Using OpenSolver page. However, if you still have a question, please post it below.

## Building Linear Programming models

Writing optimisation models that only use linear mathematical equations and inequalities is not easy. However, most of the time you want to build these “linear programming” models (and avoid non-linear models) because these are easier and more reliable to solve using packages such as OpenSolver. (OpenSolver uses CBC by default to solve these linear programmes.) Check out this Wiki page on linear programming.

The AIMMS folks have a great online chapter describing how to take non-linear requirements (local copy of pdf) and model them using linear programming. (Thanks to Stu Mitchell of PuLP fame for pointing out this resource.) The NEOS guide is also a great resource. If you still have a modelling question (e.g., how do I express this requirement using the rules for a linear program), please ask it on https://www.or-exchange.org/.

## Common Software Problems

Excel can have problems dealing with add-ons. Visit http://www.add-ins.com/how_to_repair_office.htm for a guide on fixing these.

### 1. OpenSolver not loading – July 2016 Office update (no error message)

In July 2016, MicroSoft released an update to Office 365 which prevents OpenSolver from loading unless the .zip file is “unblocked” before the files are extracted. (We expect this change to appear in other versions of Excel.) The symptoms are simply that OpenSolver does not appear in the Data tab; there is no error message shown. To fix this, delete your old OpenSolver files (but not the downloaded .zip file), right click the downloaded OpenSolver .zip file, choose Properties, and click the Unblock button which will show if the file is blocked. Once the file is unblock, close the properties dialog, and then un-zip the files and open up OpenSolver.xlam as usual.

### 2. Unable to find the external solver …

If OpenSolver reports this problem, then it could not find a solver file that comes with OpenSolver. Please check that you have extracted (i.e. un-zipped or uncompressed) all the files from the OpenSolver download.

### 3. The solver did not create a solution file.

If this error is reported, then it is likely that OpenSolver was unable to run the solver executable. This may happen if the OpenSolver files are in an ‘untrusted’ location, and so Windows will not let them be run. First check whether the solver is shown as found in the “About OpenSolver” form. If not, go to the folder identified in the “About OpenSolver” form and double click on this solver file. You should then see a command window open that gives the solver name and version information etc. If you don’t see this, then Windows should give you an error message to help you diagnose the problem. It may help to move the OpenSolver folder (and all the files it contains) into a location such as your Documents, or even into Program Files, and try again.

### 4. Solver crashes; OpenSolver reports no solution file

The files in the “Solver” folder provided with OpenSolver contain the actual solver code. These solvers are being continually upgraded as bugs are reported and fixed. If OpenSolver reports that no solution file was generated, then the solver may have crashed. This can happen on big problems when the solver runs out of memory, but also on smaller problems that happen to hit a solver bug. In such cases, you should check if a newer version of OpenSolver is available since we regularly update the solvers included with OpenSolver as they are released.

With CBC, a typical OpenSolver error for thois problem message might be: ‘The CBC solver did not complete, but aborted with the error code -1073741510’

If needed, you can download the latest version of CBC, Bonmin or Couenne from AMPL. Download the either the Windows or Mac version as appropriate. For Windows, get the 32-bit version if you have 32-bit Windows, or the 64-bit version otherwise. (The 64 bit version will solve much larger problems than the standard 32 bit version included in OpenSolver, so always choose this if possible. Right-clicking on Computer, and choosing Properties, will tell you if have a 32 or 64 bit system type.) Right click on the downloaded .zip file, choose Properties, and click the Unblock button if there is one. Then, open the .zip file, and drag the files into the right location in your OpenSolver Solvers folder (the win32, win64 or osx folder as appropriate), replacing the existing files. This may fix your problem.

**Report a CBC bug**

If after installing the latest version of CBC, CBC still crashes on you, then you may wish to report a bug. Please be sure to include the .lp file produced by OpenSolver; you can open this file using “View Last Model .lp File”. CBC bugs can be reported by either:

- The online CBC TRAC bug reporting system (which you will have to register for), or
- The CBC mail list

**Switch from CBC to Gurobi**

Since August 2014, OpenSolver has also supported solving models using the Gurobi solver. You can obtain a copy of this solver at no charge if you are at a university. Gurobi is faster and more stable than CBC, and comes highly recommended.

### 5. Repairing Excel

Sometimes it is useful (if all else fails) to repair your Office installation:

**Windows**

See these Microsoft Office 2010 and later instructions (or in Excel 2007, use Excel’s application menu, then Excel Options, then Resources, then “run Microsoft Office Diagnostics”). You might also want to look at http://support.microsoft.com/kb/166273, which talks about this.

**Mac**

Unfortunately there is no easy way to repair the Office 2011 installation. The best option is to follow these instructions to completely uninstall Office. You can then install Office again from scratch to get a clean version installed.

### 6. ‘Can’t find project or library’ : Missing Reference Errors

OpenSolver uses the “RefEdit” control, which we need but which can cause problems because of missing references caused by version changes. To check for missing references, using the VBA editor (which you access using Alt-F11), select OpenSolver (openSolver.xlam) in the Project window, choose Tools… References, and look for a “Missing:” line. If you see this, un-check that line, close the dialog, and choose File … Save OpenSolver.xlam to update the file. If this doesn’t help, try the above steps for repairing Excel.

**7. Error 70: Unable to delete the file: C:\Users\XXXXXX\AppData\Local\Temp\log1.tmp**

This errors happens if Excel crashes while OpenSolver is running, leaving log1.tmp open. Please restart your computer, which will close this file, and try again.

**8. OpenSolver was working on my Mac. Then I updated to Excel 16.16.7 (released late 2018) and OpenSolver stopped working. What do I do?**

We have had numerous users report this issue. We are looking for a Mac user with VBA experience to help us diagnose this problem. Please contact us at email hidden; JavaScript is required if you can help.

**9. OpenSolver does not work in Excel installed from the ****Windows Store**

Windows Store versions of Office applications do not support Add-ins such as OpenSolver; see this link, or this support page for another add-in, or this technical article from Add-in Express. This Dell article explains a little more. If you have a Windows Store version, it will display this in the Excel Version information, such as in the following example:

Version 1807 (Build 10325.20082 Microsoft Store)

The solution is to un-install Excel, and install the click-to-run versions, in which case the Excel version will change from ‘Microsoft Store’ to ‘Click-to-Run’. Standard installs (meaning not Office 365) don’t have this issue.

I have a knapsack-type problem where I choose 3 items out of 100 to maximize the sum of two independent values found on each item that have diminishing returns. Therefore I must use a non-linear solver.

When solving my problem without the diminishing returns piece and using a linear solver, the solution works great. I’m currently getting the error “Non-linear parser failed while processing cell Sheet1_D102”. I’m curious as to why it’s getting hung up on cell D102 since it’s the sum of the binary column to determine which rows (items) should be selected. The constraint is that this column should sum to 3.

Hi,

I’ve set up the quick solve model, and it is working. Save my excel, and then reopen it to run quick solve, but then I need to re-run the full model for quick solve to work… Can’t I somehow save the model and be able to quickly run it when I reopen the excel?

Thanks for your help,

Kr,

Delphine

OpenSolver currently has no way to save the Quick Solve data, sorry. If you are a programmer, we would welcome you adding this feature. Andrew

After double clicking opensolver.xlam I see that excel is loaded but then nothing happens.

I am using windows 10 and tried also to place the files under …\microdoft\addins

Any advice is welcome

Hello,

I am working with a 3900-4000 variable MILP that comes up with an optimal solution. When I extend the variables to say 4200 variables, I get an error that OpenSolver comes up with an infeasible solution. Is there a way to find out what constraints are making my program infeasible now that I have added a few more variables?

Not automatically; you will need to add extra variables to let constraints be violated, eg replace x+y<=10 with x+y<=10+z, and then penalise the sum of these extra variables. Hope this helps, Andrew

Thank you for your response Andrew.

I have extrapolated my MILP to around 30000 variables now, and it has found an optimal solution after around 104000 s, or 29 hours roughly. With 4000 variables it used to take like 10-15 minutes.

I believe I am running it with a pretty decent computer system, which is 12 gb ram quad, between 3.1-3.5 GHz processor. Any ideas to reduce the solving time on my MILP? As long as I can get it to solve within a few hours at most I should be good.

Many thanks!

Aaron

If the time is spent extracting the model, then try looking at SolverStudio. If the time is spent in the solver, try increasing the optimality tolerance, or switch to a commercial solver like Gurobi. Andrew

I have been using OpenSolver for a few years and have learnt a lot about linear programming in that time. I have extensive Excel experience and find structuring the models quite intuitive in OS. However I am now at the point where my models are becoming too complex for OpenSolver taking many days to solve in some cases. I am trying to take the step to Solver Studio however I am finding it very difficult to build the equivalent models in Solver Studio (in PuLP). Is there any way to convert an OpenSolver model into a Solver Studio model or maybe some type of code output of the OpenSolver model that could help in this conversion?

Great that you are trying SolverStudio. Sorry, but there’s no shortcut for this move from OpenSolver to SolverStudio. However, you may find it useful to export the .lp file from OpenSolver, and look at this to better understand your model. Andrew

Hi Andrew,

Quick question: How do we export the .lp file from OpenSolver ?

Hi, I am trying to build a linear model for sequencing 3 products on assembly line. The goal is to maximize the amount of same product in a row. In other words, minimize the number of product changes. Non linear model works easily for a few variables (excel solver) but I need to create a sequence of 1400 units. Is there any simple way, how to express this problem in a linear way? I am trying to use binary varibles.

Open solver seems to be the perfect tool for this task. Thank you

Hello,

I received this error (The Couenne solver did not complete, but aborted with the error code -1073741819). after OpenSolver finishing constraints and variables. my model is quite large, it has 3264 binary variables and 1632 continuous variables (4896 in total). anythoughts, please.

Hi,

I am trying to use your OpenSolver with Google Sheet and the add-ons installs alright. Launching the OpenSolver within the Google sheet starts up and after some it seems to be ready as the text turns from grey to black. However, it is not possible for me to fill anything in the fields. It is not possible for example to fill in the sheet name or press the button “Current sheet”. I have tried to change the language settings for the sheet and have also lokked at the security setting for my account but couldn’t find anything that would get it working. Do you have any suggestions I should do?

Best regards Tomas

Google seems to have changed something that has broken OpenSolver. We don’t have the resources to keep tracking down issues caused by Google;. We don’t have a timeline on when this will be fixed, sorry. Andrew

Hi! My name is Julieta, I´ve been trying to use OpenSolver with GoogleSheets but for some reason I can´t fill in the cells, it´s kind of blocked, locked to write. I will appreciate your help. Regards, J

Sorry it is broken; Google has changed something. We would welcome community support to fix this. Andrew

Used this tool in Excel on a Mac for years and it’s been incredible useful. Recently had to switch to a PC and can’t get Solver to work. I’ve extracted all the files into a folder on my desktop, open the “OpenSolver” xlam file, then open the Excel file I’m working on (saved in the same folder), but no Solver tools show up in the Data ribbon. Open Solver Version 2.9.3, Excel version 2014 Build 13929.20296 Click-to-run. Anyone have any insights or similar experiences working through this?

Hello,

I installed OpenSolver as an Add-on to Google Sheets but it seems to not work. Thus, while opening OpenSolver at the side bar, it would not even let me choose a sheet or a range, so that I cannot do anything with this feature.

A screenshot of the issue can be found here via WeTransfer: https://we.tl/t-5UPD2njKMU

Could you please look into that?

Thanks,

Ron

Google has changed something. We’d welcome support from our community to debug and fix this. Andrew

I am using 2.9.0 in a spreadsheet. Want to update to 2.9.3, but also want to change where OpenSolver is stored. How do I do this?

Just put OpenSolver where ever you like, and open the OpenSolver.xlam file it when you need it; no need to load automatically at startup. (I pin my OpenSolver to my recents menu.) Andrew

I’m currently using the COIN-OR CBC (Linear solver) but I’m not getting an optimal answer. After doing some research it seems my model may not be linear because of all the binary variables I have. Would it be better for me to use a non-linear solver? And if so what do you recommend?

You can try a non-linear solver, but I suggest you first try to make your model linear. Linear problems are normally easier to solve. Andrew

Is there any way to install IPOPT into open solver please? I was looking an engine to solve local optimal solutions. Thank you

IPOPT is already available, embedded in COUENNE and BONMIN. Andrew

Hi Andrew, Thank you for your time and reply. How can I ask the opensolver to only find the local optimal solutions pls? Currently, the solving time of the engine is even slower than the excel solver.

Thank you

What can I do to solve this error?

“Microsoft Visual Basic for applications

Object Library invalid or contains references to object definitions that could not be found”

Try repairing Excel. Andrew