Unlisted screen resolution
first run
$ xrandr
this will give output like this:
Screen 0: minimum 320 x 200, current 5120 x 1080, maximum 8192 x 8192
HDMI1 connected 2560x1080+2560+0 (normal left inverted right x axis y axis) 677mm x 290mm
1920x1080 60.00*
1680x1050 59.88
1600x900 59.98
...
HDMI2 connected 2560x1080+0+0 (normal left inverted right x axis y axis) 677mm x 290mm
2560x1080 60.00*+
2560x1080_60.00 59.98
1920x1080 60.00 50.00 59.94 30.00 29.97
1920x1080i 60.00 50.00 59.94
...
to add 2560x1080 to HDMI1 first run the next command
$ cvt 2560 1080
sample output:
2560x1080 59.98 Hz (CVT) hsync: 67.17 kHz; pclk: 230.00 MHz
Modeline "2560x1080_60.00" 230.00 2560 2720 2992 3424 1080 1083 1093 1120 -hsync +vsync
now you need to use this output to test the resolution and than add the settings for use in the next sessions.
testing:
$ xrandr --newmode "2560x1080_60.00" 230.00 2560 2720 2992 3424 1080 1083 1093 1120 -hsync +vsync
$ xrandr --addmode HDMI1 2560x1080_60.00
$ xrandr --output HDMI1 --mode 2560x1080_60.00
if it works you can add it to the xorg configuration
$ sudo vim /etc/X11/xorg.conf.d/40-monitor.conf
or
$ sudo vim /usr/share/X11/xorg.conf.d/40-monitor.conf
Section "Monitor"
Identifier "HDMI1"
Modeline "2560x1080_60.00" 230.00 2560 2720 2992 3424 1080 1083 1093 1120 -hsync +vsync
Option "PreferredMode" "2560x1080"
EndSection