KB: Understand Spice models supported in Altium

Solution Details

Largely, there are two main SPICE versions commonly available, SPICE2 and SPICE3, while below is a more elaborate breakdown: 

• SPICE3 (Berkeley): Spice 3F5 being the final version

• XSPICE (Georgia Tech): Extension based on SPICE3

• NGSpice (open-source): successor of Berkeley Spice 3F5, XSPICE, and Cider1B1

• LTSpice (Analog Devices): SPICE3 evolved and modified with some custom/proprietary extensions by LTC, later acquired by Analog Devices

• PSpice (MicroSim/OrCAD/Cadence): SPICE2 evolved and modified with some custom/proprietary extensions by MicroSim, later acquired by OrCAD and then by Cadence

Altium Designer supports SPICE2/3 and XSPICE.  Starting with Altium Designer 21, some support was added for both PSpice and LTspice model variants: 
https://resources.altium.com/p/altium-support-pspicer-mixed-signal-circuit-simulator

A device manufacturer site is the first place you may look for a model of most accurate representation, but for a simple/casual model, you may opt to create one yourself.  Prior to Altium Designer 21, it was necessary for Spice models to have the correct file extension to be recognized. New in AD21 is a *.LIB model file extension. This will recognize both .MODEL and .SUBCKT definitions in the file. The older file extensions will still be recognized: 

https://www.altium.com/documentation/altium-designer/verifying-preparing-project-simulation#!adding_simulation_models_to_the_design

For a casual, analytical models, you may find Simulation Generic Components.Intlib convenient, installed by default at the path C:\ProgramData\Altium\Altium Designer {GUID}\Extensions\Mixed Simulation\Library.
Please note that all other legacy components bundled previously has been consolidated here starting in AD23.3:
https://www.altium.com/documentation/altium-designer/new?version=23#!key_highlights_23_3:~:text=Removed%20Default%20File%2Dbased%20Libraries
But can be downloaded indepedently as needed from here:
https://www.altium.com/documentation/other_installers#!altium_designer_23-2_libraries

Our long-term intention is to have spice models linked upfront to wider range of components you can save/download directly from Manufacturer Part Search panel:
https://www.altium.com/documentation/altium-designer/verifying-preparing-project-simulation#!sourcing-a-simulation-ready-component-from-the-manufacturer-part-search-panel

Encrypted PSpice/LTSpice/HSpice/TINA-TI models:

Models encrypted for a particular simulator can be used only in that simulator strictly. Device manufacturer encrypts its model for the purpose of protecting their IP, so it can be decrypted solely by the known designated tool.
There is a BC to have Altium work with device manufacutrers to establish equvalent encryption scheme for Altium, that you may upvote/follow to get any update:
https://bugcrunch.live.altium.com/#/idea/7123
In the meantime, the only workaround conceivable is to find another equivalent/similar model or to negotiate with the manufacturer to obtain a decrypted model.

Unsupported model type 'A' error on LTSpice models:

According to the syntax of SPICE simulators, each instance of a primitive model must begin with some reserved Latin letter.
http://www.ecircuitcenter.com/SPICEsummary.htm

LTSpice is an extension of the Spice 3F5 language and has a number of additional primitive models, the description of which begins with the letter "A".
https://ltwiki.org/?title=Undocumented_LTspice#A-Devices
https://ltwiki.org/LTspiceHelp/LTspiceHelp/A_Special_functions_.htm

Likewise, the XSPICE extension underlying the Altium Mixed Sim simulator also has a number of primitive models with instance descriptions beginning with the letter "A".
https://techdocs.altium.com/display/AMSE/XSpice+Models

And here is an important point - sets of additional LTSpice and Altium Mixed Sim models do not overlap. This means that there are some LTSpice models that do not exist in the Altium Mixed Sim. Among the models not implemented in the Altium Mixed Sim are OTA and SCHMITT models.  These are coded LTSpice models, which means it cannot be imported into Altium Mixed Sim as the code remains proprietary to Analog Devices, Inc.